# *90*
Hi NorthWest
>> that's probably a part of why a picture of one is on two album covers..<<
Oops ? which one's ?
Niagara dual headlights : .. and they were *so* "showing the way" ( ALCO advertising ALCO ) towards beloved typical American automobile twin headlights on Mopar , Ford and Giant Mommy full size sedans of the late 1950s through '60s and '70s ..
Here's another one of my T1 variations , T1c , again in a twilight of steam 'ingloomination' when great engines became heroic as they kept soldiering on through increasingly adverse times ...
Side remark : light setting may or may not accord with actual exposition of Englewood - however point is the setting-sun evening light *as such* symbolizing transition in a more metaphysical way
With regards
Juniatha
#91
Hi Juniatha!
The albums:
http://en.wikipedia.org/wiki/Movin%27On(Commodores_album
http://en.wikipedia.org/wiki/A_Different_Kind_of_Truth
I guess we'll have to disagree on the Niagara late sealed beam headlights, they just say "diesel switcher" to me (picture: http://en.wikipedia.org/wiki/File:NYC_Niagara_1.jpg)
The T1c looks great, thanks for eliminating that horrid running board step!
NW
# 92
Albums - Ok , now I see why I didn't know them :
Commodores - commonly not played on my table ;
VanHalen - Jeezus , uhm , yeah - uuh- no , thanks !
btw that's a real weird version of an X-whatever typo loco , see
http://www.youtube.com/watch?v=BMkFmc1_90E
( sorry , never mind in case they should be your favorites )
We don't disagree on the dual headlight - it was sarcasm allegedly it wold have lead to the dual headlight on later sedans - of which one at least *was* in fact named LTD ( Ford ) and thus seems to have closed the cycle - *g*
Regards
#93
Juniatha, thanks for the link! They actually did a good job with the opening animation, I was expecting something more along the lines of Stephenson Valve Gear when I opened the link .
About your locomotive revisions-what software are you using?
Thanks
# 94
Note the exposed firebox, like I wish to do to your and Dryfuss' J3a. And the runningboard straight to the front steps.
#95
Ref #86:
Juniatha About the Allegheny : Today very much the same engine - boiler dimensions , pressure , cylinders d x s , drive wheel diameter et all - could be manufactured to about 24 - 26 t axle load limit while the original's was 39 t [metric each] . Yet , that would inevitably affect the engine's adhesion limit tractive effort , same cylinder tractive effort notwithstanding . Or in other words - it would tend to become rather a very powerful and big passenger train engine - especially if steam circuits would be revised ...
About the Allegheny :
Today very much the same engine - boiler dimensions , pressure , cylinders d x s , drive wheel diameter et all - could be manufactured to about 24 - 26 t axle load limit while the original's was 39 t [metric each] . Yet , that would inevitably affect the engine's adhesion limit tractive effort , same cylinder tractive effort notwithstanding . Or in other words - it would tend to become rather a very powerful and big passenger train engine - especially if steam circuits would be revised ...
True...though I would just advocate bringing the weight down to what they were *supposed* to be when built. (Anyone know what that was? I remember there was a scandal when they were overweight...)
Although, intermodal trains, like some of BNSF's UPS Z trains, run consistently at 79MPH. So, once you got it started, would a big passenger engine be not so bad?
# 96
Though I don't have specific data on what the agreed maximum was for powered axle load , I would roughly put it in the vicinity of 80000 lb . Again without specific information , it would appear many railroads have since eased down on maximum axle loads since it had proven in early dieselization it was not advisable to load small diesel wheels the same as larger drive wheels in steam locomotives and then without the 'introducing' lighter loaded leading idler axles of steam locomotives . So , to be sure I would still want to aim at 66000 lb , preferably lower , in a today's classic steam locomotive design .
View on permissible axle loads was largely differing in the US from what was - still is - practice in Europe : while in the US it would appear many RRs permitted high loads on drive wheels of large steam locomotives - then again presenting a considerable congregate vehicle mass - while permitting much lower loads on freight car and passenger coaches axles - or was it simply for freedom of exchange and universal reach ? can't imagine that was the only reason - in Europe what axle load was permitted was permitted if you want from first to last axle in a train - no extra allowance for steam , much as steam designers would have liked it and may have envied their American colleagues !
Presently , although much of mainline track is laid with 43 lb / ft rail and on very solid banquette with steel reinforced concrete sleepers , continuously welded of course , general axle load limit is 22.5 t [metric] i e 50400 lb .
Still, that seems enough for railways having virtually banned six axle electrics not just from construction but from rosters , meanwhile . In the Sixties , every national railway that was asking plenty of power from their modern electric traction had to have their Co-Co types – Austrian ÖBB for the Alpine mainlines , obviously , same with the Swiss SBB-CFF including Bo-Bo-Bo variations , W-German DB with their 150 class freight , then when they introduced 200 km/h service the w/a went express power , too ; likewise Co-Co electrics roamed the French SNCF , naturally in various variations ( shortest joke on locomotive standardization , just one word : SNCF ! ok , could be spelled DBAG , too , these days ) Now , there is but one w/a ruling and that is the universal Bo-Bo type .
Classic steam invariably tended towards speed to unreel power – that would still even more be true if really up to date classic steam locomotive types were to be built : most all advance in power would go into speed – so , a hypothetical railroad returning to steam on account of “ Hey , we have coal available in abundance , we want to get rid of the oil price lottery and we are prepared to spend some fuel on somewhat reduced thermal efficiency – not downright low as it was back then !” would have to turn to a dramatically different way of rolling their tonnage – rolling is the word ! rolling thunder it would have to be , avoiding extreme tonnage , freight trains would pass at sustained 60 – 80 mph on trackage of mint alignment , GPS overviewed and signaled to run at incessant follow-up succession with conveyer-belt reliability and precision . Overall tonnage delivered every 24 hours could unquestionably be fully up to diesel traction and competitive in a traffic shifting sense . It would involve more personnel overall – yet if you have a busy line connection and you’d attract good customers by consistently doing top service : you can’t save as much as you can earn more by a clever business concept …
A look at what has actually been going on where the original was to be duplicated may provide a glimpse. Climax #6 on the White Mountain Central will be case in point.
This 50 ton B-B Climax locomotive was built in 1920 as the last piece of motive power for the Beebe River Railroad and used to log some of the very last virgin spruce stands in New Hampshire. It was run until the gears would hardly mesh and was put away in 1927. It was purchase at scrap price in 1950 as the first of a series of geared and rod locomotives and was steamed after 30 years on Christmas Day of 1957. The White Mountain Central was created the next year but this engine was not put to use immediately. But as traffic increased first the 25ton 0-4-0T Porter was replaced with a completely rebuilt 35ton Heisler and when trains got up to 5 cars, the Climax gears were replaced. This continued until a few years ago when it became obvious that the boiler needed serious repair. The owners looked carefully at the impact to their mainstay motive power and decided on total replacement of the boiler rather than repair. A couple of years later the engine's crankshaft failed and had to be replaced. In each case the replacement was clearly higher quality even though it was as close to a duplicate as possible.
This locomotive and its current engineer have been together many times more than the years spent in its original purpose. But there is the constant possibility that increasingly tightened regulations may force and end to this tourist railroad that is a sort of operating logging museum.
My last should have been #97
#98
On the other end of the spectrum, whatever comes out need to move in the direction of:
1. Seriously higher boiler pressure
2. X-X-6: six wheels under the firebox
3. Better steam distribution: better valve gear, uni-flow path for reciprocating
4. Move toward turbines. Ships did it. Locomotives need to as well, whether direct connected or otherwise.
5.Use of powdered coal similar to coal fired utilities using high tech air mixing nozzles.
6. The coal-fired "gas" turbine is no longer out of the question now that the Shell-patented separators are allowing power recovery trains from catalytic cracker blow-downs.
I find too many people involved here in thinking within the railroad box and not aware of other industries.
Steve
#99
This (#98) raises an interesting question that was not really part of the original thread (Juniatha has other threads, of considerable interest, regarding advanced steam concepts, which should also be bumped!) This one involves, as the previous post (#97) addressed, operation of legacy locomotives in modern conditions.
I suspect #1 is not a particularly good option in any legacy context, and particularly for any locomotive retaining a water-leg firebox without active circulation. Thermodynamically, of course, increased pressure is an advantage -- just a relatively minor one for typical locomotive operations, and at a relatively outsized maintenance (and, pun intended, insurance) premium.
Most of the issues with higher pressure are not directly related to riveted construction, so while all-welded construction with any cost-effective modern steel would help somewhat, it's not likely to be enough. We could take up the interesting discussions concerning best staybolt practice (for example, welded a la Tross, with the inside sheet profiled following the bending-moment diagram) but that is really more of a discussion for modern steam technology, not historic preservation.
#2 -- most of the late European improvements went in the opposite direction; the 242 A1, for example, had a four-wheel trailing truck just for weight-distribution reasons. As it happens, I support the idea of large grate area on some types of modern locomotive, but in all probability anything non-articulated will be perfectly satisfactory with no more than a four-wheel trailing truck. I have not run calculations on the mass distribution for the double-Belpaire boiler, but I doubt that with modern axle loading and wheel steels it would be high enough to mandate an extra trailing-truck axle over what would otherwise be needed, even with all the mass of the syphons in addition to Cunningham circulators.
#3 -- Can you be more specific about what you mean by 'better', with examples?
I, personally, still like the idea of poppet valves, and as it turns out most of the 'bugs' in the detail design of the Franklin System appear to have been resolved as early as 1948. On the other hand, `there is a great deal of specialized construction required to implement and to maintain a poppet-valve (or even a drop-valve) setup, even before we start the discussion on continuous-contour RC cams in a locomotive environment. I think it is notable that David Wardale is a proponent of improved piston valves (with radial drive) even in an era of sophisticated and easily-CAD/CAMmed and lost-foam-cast poppetry.
Uniflow has a long and fairly disastrous history in locomotive practice, and there is in my opinion little to recommend its adoption in any modern locomotive. About the only detail that bears copying is the self-aligning LP seat on the Skinner Unaflows with LP poppets ... and that does not relate to the exhaust method. In order to get the compression anywhere near where it needs to be for high-speed running, you need auxiliary exhaust ports -- which probably need to have both timing and duration separate from the inlet valves. You have added a very long piston, acting entirely as reciprocating mass, which makes balance a problem, acting in a very long cylinder casting, which will probably increase locomotive length and complicate the design and implementation of the leading truck.
Uniflow makes much better sense if you are building something like a Paget locomotive, or one of the flavors of motor-locomotive. But that is not what most of the locomotives concerned in the present thread would be using...
#4 -- I happen to agree in principle with this, in at least two ways. I was a strong supporter of Alan Fozard's Turbomotive 2 proposal, back around the turn of the millennium, and I have done some careful initial research into the PRR V1 turbine (which started as a noncondensing locomotive with two mechanical-drive turbines, and then was to be given a variant of the Bowes ship drive.
Again, these would involve significant cost and eliminate some of what makes the steam locomotive attractive to enthusiasts -- in the case of the V1, most of it. The big issue is the effective water rate, which even with the implicit multiple-stage 'compounding' implicit in multistage reaction turbines, is excessively high at low to moderate speeds (if some variable-speed transmission isn't used). This is one area where the Bowes drive can help considerably... just not enough to make up any flexibility difference between the turbine and a good positive-displacement expander.
I won't take up the effectiveness of attempting to condense the exhaust from a 4000-hp-plus locomotive, except to note that railfans aren't likely to favor the result with their fantrip dollars for very long. You'd probably be better off by improving the Rankine-cycle efficiency of conventional locomotives, and use multiple auxiliary tenders.
#5. Every time this has been tried on a locomotive, sooner or later there will be a critical-mixture explosion. Everybody knows explosion doors don't work. It's cute to call a delayed-ignition event a 'puff' but its sequelae on the road aren't going to be quite as containable as they are in a stationary plant. And all this before we get into what happens when the pulverizer jams, or the hot-air feed stops working properly, or the flameholding cuts out, or the balance between forced and induced draft goes transiently out of whack...
PC firing is a comparatively advanced technology, as is the incorporation of torrefied and other biomass fuels, and steam injection to reduce NOx (with a long enough, or effective enough, gas path). The issue is that applying it, PRACTICALLY, to a locomotive expected to earn its keep and not fail on the road even under potentially severe conditions, is not easy, and other alternatives are in my opinion far superior.
#6 -- the issue with the coal turbine was never that separation could not be made effective; it was that 'outliers' in particle size, either from improper crushing or ash aggregation, could not be separated within the available packaging dimensions and with the required TOF of the particles at locomotive firing rates. If you can be specific about how the Shell separators have addressed either of these concerns (and especially if you will provide the patent reference for them) it would be a great help to me. I do have to suspect that if you reduce one of Shell-s separators to the dimensions required in a coal-turbine combustor, and then apply the required firing rate to it, there may be... concerns.
Meanwhile, how were you going to solve the particular problems with ash on the 'other end' of a coal turbine? The ash on the B&W boiler in the N&W TE-1 was already notable as having an irritating 'fines' content (according to Louis Newton, who should know); a high-throughput combustor (with corresponding very high volume of exhaust gas at high speed), even if you posit the advantage of some kind of Rankine-cycle bottoming, is still going to cause some containment difficulties on something the size and dimension of a practical road locomotive.
Many of us here are well aware of solutions and technologies in other industries, and have been for decades. The absence of what appears to be innovative proposals does not mean the absence of knowledge, or the existence of hide-bound conservatism and rejection of modern technology and engineering practices (insert other condemnation as appropriate! ;-} ). I have repeatedly found, however, that where an apparently attractive technology has remained unused, there are usually either practical or economic reasons... and often, both.
Railroads pose special design conditions, and not only because of the kind of people and problems involved with them. Be sure that when you design for them, the result is going to be effective in their specific context.
#100
I just HAD to have #100.
This comment deserved more attention and respect than it seems to have gotten so far.
JuniathaClassic steam invariably tended towards speed to unreel power – that would still even more be true if really up to date classic steam locomotive types were to be built : most all advance in power would go into speed – so , a hypothetical railroad returning to steam on account of “ Hey , we have coal available in abundance , we want to get rid of the oil price lottery and we are prepared to spend some fuel on somewhat reduced thermal efficiency – not downright low as it was back then !” would have to turn to a dramatically different way of rolling their tonnage – rolling is the word ! rolling thunder it would have to be , avoiding extreme tonnage , freight trains would pass at sustained 60 – 80 mph on trackage of mint alignment , GPS overviewed and signaled to run at incessant follow-up succession with conveyer-belt reliability and precision . Overall tonnage delivered every 24 hours could unquestionably be fully up to diesel traction and competitive in a traffic shifting sense .
Wardale, in the Red Devil book, goes to some length about how the ACE efforts 'should' have been directed toward producing peak locomotive power in the appropriate speed range for the anticipated traffic. It is perfectly practical to design freight power to 'peak' in the 45-to-55 mph range required for current coal-train service, and get faster and prompter acceleration in the process. Would that not provide locomotives better suited to modern operating conditions in the United States?
Some of the discussion involving the duplex-drive Q2s indicated that a smaller driver diameter would have been a substantial improvement, given existing PRR operations. That is just one example of how optimization for slower average freight operations might be made. Not that I'd object to faster operations!
For an interesting 'take' on this operating concept, see the post on 'future railroading' that Don Oltmann put up in his blog, referenced in the post over in General Discussion on "PTC + ECP + DPU - Coal + Intermodal = ?"
locobasede # 2 Finally, I'd really like to see a truly clean-burning coal-fired design, or maybe we could get McDonald's to sponsor one fueled used french-fry oil.
# 2
Finally, I'd really like to see a truly clean-burning coal-fired design, or maybe we could get McDonald's to sponsor one fueled used french-fry oil.
Out in Colorado, they have an old steamer running on CANOLA OIL!!! Believe it or not, it's true. This reminded me of the McDonald's french-fry grease thing, because the crew often describes the scent of the burning oil as "French-Fry scented". Just thought that should be thrown out for my own purposes.
Hello folks ,
dunno - to me it seems hard to see what "smaller drivers on a Q2 for better service on PRR" (oops ??? - at 69 " they already were smaller than those of the N&W A which obviously had no trouble with 70 ") and "CANOLA OIL" ( my goodness ) have to say for operation of classic type steam locomotives in the 21st century ?
= J =
Juniatha Hello folks , dunno - to me it seems hard to see what "smaller drivers on a Q2 for better service on PRR" (oops ??? - at 69 " they already were smaller than those of the N&W A which obviously had no trouble with 70 ") and "CANOLA OIL" ( my goodness ) have to say for operation of classic type steam locomotives in the 21st century ? = J =
Well I have to say it's better than to leave the thing to rust out by some park or other.
Getting back to the original posting, I think future efforts should focus on the smaller 4-4-0 engines. They are much smaller and more versatile than the larger engines, but just as impressive. They're also cheaper to build and run than, say, a Big Boy. There's a guy in IL by the name of David Kloke who has built a couple of examples brand new. Beautiful engines.Google David Kloke if you haven't already done so... his craftsmanship is something to behold.
#105
Juniatha ... to me it seems hard to see what "smaller drivers on a Q2 for better service on PRR" [has] to say for operation of classic type steam locomotives in the 21st century ?
This was in reference to the idea of operating steam for freight service, rather than special passenger or enthusiast runs.
PRR had a relatively low freight speed limit, as noted, and more people than I have commented that locomotives with smaller drivers would have made good sense given that constraint. However, transitioning to 21st-Century conditions, there are many cases in modern railroading where restricted speed (say, to 45 mph) is adequate to requirements, particularly if 'one-speed' operation around that speed can be provided.
Given that restriction in road speed, shifting the HP peak of the locomotive to fall no higher than the desired range seems sensible, and one good way to achieve this is to reduce driver diameter, ceteris paribus (assuming, of course, that augment and inertia forces are controlled even at highest service speed). This is also likely to increase acceleration rate over a wide range of service speeds.
That's really all the PRR reduced-driver-size comment was intended to refer to, in context. It was intended analogically, rather than literally.
But remember that for many years speed limits on the PRR were truly a legal fiction, with T-1's being goosed up to 120mph on passenger trains and freigihts behind M-1 4-8-2's regularly hitting 70mph beween Crestline, Fort Wayne, and Valpariso. And everyone knew what was going on; it wasn't really a secret.
Guys
Whatever the speed limit was and whatever actual speeds or 'speeding' in this case , this is topical of PRR train handling and since PRR steam traction has long since ended and the RR itself has long since ceased to exist for sure this is *not* topical in this thread .
Overmod : ohh-kayii , if you insist on it : por favor would you mind to specify about your smaller wheel diameter was it -
-a- meant to go with same dxs cylinders , same bp - or
-b- with reduced bp or / and stroke according to reduced wheel diameter ?
?
Hi Ulrich
oh , hey - well that's something ! cute-nice for a small haul tourists line for example .
Yet , I'd like to see a 'modernized' 4-4-0 built with Walschaerts and piston valves for high superheating - *gee* - possibly oil-fired - or actually LNG-fired , too .. 2000 ihp straight from the cylinders , chee-chee-chee ..
#109
Juniatha Overmod : ohh-kayii , if you insist on it : por favor would you mind to specify about your smaller wheel diameter was it - -a- meant to go with same dxs cylinders , same bp - or -b- with reduced bp or / and stroke according to reduced wheel diameter ?
As a merely illustrative example, and moreover only an analogy, it really doesn't matter other than 'smaller' (essentially meaning higher cyclic at a given road speed). As you and I agree that using the example of a Q2 means nothing in the modern context; the bore, stroke, and probably pressure and steam-circuit for a new locomotive would be designed to suit the chosen wheel diameter. Long-term strength of the wheel center, especially at the axle and pin fits, and adequate securement of the balance masses, would have a great deal to do with determination of the 'most effective' range of "lower" driver diameters.
(On a Q2 I think I'd reduce both bore and stroke on both engines, with the rough expectation of keeping the mass flow the 'same' at a given road speed, and use full lightweight rods and valve gear; it would remain to be seen whether derating the boiler pressure, building new boiler shells, etc. would have benefited a Q2 as much as a Niagara. But none of this properly belongs in this thread.)
With regard to the 4-4-0: might I suggest the Schools class as a pretty good starting point? We even had Repton here as a potential guinea pig (or firsthand source of dimensions) -- but she is now safely home and away from us "improvers"!
#110
Guess Ill add some here. For modern Passenger use , and as originally designed a 4-6-4T engine is ideal and very cost effective to build. Anthracite coal has the most BTU per ton of any carbon fuel on Earth. Adding in Uranium nitride can extract even more energy. Improving fireboxes and shakers can retain even more heat. Keeping the loco short means less wear on the tires and front trucks. Another upgrade could be to power the thing with small Thorium reactor which has been done in a car already. While not nearly as reactive as uranium, a 1lb nugget could power a 4-6-4T for 5 years in regular service under 100 miles a day.
PIc of CN 4-6-4T , only one in the world left
https://www.flickr.com/photos/milantram/3857122462/in/photostream/
There was a university trying to retrofit a Steamer for biomass a few years ago. I do know in Northern Maine where paper mills once thrived on the Bangor and Aroostook railroad, they now make Bio Coal from wood pulp. Its called Thermogen. While it would be interesting to see how it "stacks up" to real Anthrcite coal, its main market is overseas as they can afford to have the stuff made.
#111 Have you any idea of what a pain it is to develop a shield for fission spectrum neutrons that will fit in a standard loading gauge?
Hooooo boy, a steam locomotive with a nuclear reactor, thorium or otherwise, rollin' down the mainline. I can't wait to hear what the NIMBY's would have to say about that one!
#113 @ ERIKEM >>There are rectors the size of a suitcase for thorium, and have been for a few years. Aside from the "shock value" of it, the 4-6-4T loco is ideal size for new steam usage
#114 The reactor core may be the size of a suitcase, but the shield would be much larger. FWIW, I do have a degree in Nuclear Engineering.
- Erik
You know, there's a perfectly good 4-6-4T sittling idle up at Steamtown not being used, it's been there for years, sounds like someone should go up there and have a heat-to-heart with those folks and tell them of the good they can do mankind if they hand it over for experimental purposes.
I mean, come on, THEY'RE not doing anything with it!
Dream on. Not going to happen. Recently had a guided tour of the backshop at Steamtown.
Do a reality check. Go there for a tour and see what is needed and how expensive it is.
Forget opinions.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
Take it easy Rich 'ol buddy, just trying to inject a little levity here.
At any rate, I'm not saying Steamtown has to do anything with that 4-6-4T, just loan it out to someone who will, like the thorium reactor boys.
If it glows in the dark when they get it back, well, just think how much money they'll save on exterior lighting.
#118
BARFlyer(quoting #110) ... Anthracite coal has the most BTU per ton of any carbon fuel on Earth.
... Anthracite coal has the most BTU per ton of any carbon fuel on Earth.
But it's the effective rate of heat release that's the operative problem. (See Sinclair's Development of the Locomotive Engine for some of the issues and approaches).
Adding in Uranium nitride can extract even more energy.
As the orals committee said to Craig Stoll, 'could you be more specific?' How are you planning to utilize uranium nitride(s) and what do you hope to accomplish with the use of such material?
Another upgrade could be to power the thing with small Thorium reactor which has been done in a car already. While not nearly as reactive as uranium, a 1lb nugget could power a 4-6-4T for 5 years in regular service under 100 miles a day.
PLEASE tell me this has nothing to do with Charles Stevens or Laser Power Systems. Or those guys at this place, with their 'rear earth metals' and magic gas-turbine heating system
Yes, you can shield natural thorium with tinfoil, but look at its heat release rate! The classical thorium cycle involves 'breeding' Th-232 to Th-233 with neutron flux ... but that involves much more shielding. As does the subsequent energy extraction out of the Th-233 decay. It's attractive to design cycles that involve only charged particles (no primary gamma) and I'm sure erikem can explain why these are desirable for power generation -- albeit electrical generation, not just thermal release -- but implementing these on locomotives is not cost-effective, and I don't think ever will be made cost-effective. (We could discuss aspects of nuclear safety on reciprocating steam locomotives, but there isn't much point!)
I do know in Northern Maine where paper mills once thrived on the Bangor and Aroostook railroad, they now make Bio Coal from wood pulp. It's called Thermogen.
This is an example of a torrefied fuel, similar to what the Project 130 people are doing. It's primarily intended at present as an additive that improves aspects of bituminous-coal combustion -- not as a fuel in its own right (although it is substantially carbon-neutral once production has ramped up).
It would be interesting to see a detailed comparison between what's required to burn anthracite vs. various levels of torrefied product on a reciprocating locomotive. I'm not sure, however, that either would represent a practical alternative, let alone a cost-effective one, for a working six-coupled tank locomotive...
#119
If we have to discuss 4-6-4Ts, and by extension 4-4-0s (;-}), let's at least raise the operative issue with respect to mainline operation. Note that this involves operation in the United States, and not in Britain or Europe where both conditions and historic locomotives are different.
I don't recall if the issue of cost-effective operation size has been brought up in this thread, and a quick review didn't seem to show it there. The cost of insurance coverage alone puts a hard limit on how 'small' a mainline operation can be, and therefore how many tickets need to be sold, cars provided to carry the ticketholders, and so forth. Meanwhile, increasing congestion and, perhaps, one-speed or fleeting operation probably require reasonable accelerative power with the required longer consists, and while this *might* be achieved with light high-horsepower locomotives, with correct train-handling skill, I think it is increasingly unlikely that the right skills would be consistently applied in the 'right places' and that confusion and delay would often result.
All this within the constraint that operation provide minimal track damage, problems for other trains, or other problems for potential 'host' railroads.
This suggests a middle ground for continued main-line steam operation, in a locomotive size that supports a healthy number of potential excursions per year on an ongoing basis, including the continued appeal needed. What do we think that 'practical' size range should be?
It's been mentioned that a rebuilt K4 may be too small to accomplish this (but is too large for practical operation on a shorter tourist line (say, Strasburg) in between mainline excursions. On the other hand, I firmly believe something like a replica NYC 5344, fitted with a more modern high-speed reversible booster, would be fully capable. I find it particularly interesting that the 'new' NS steam program finds value in putting a feedwater heater on a 'legacy' 2-8-2 for excursion service.
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