Overmod
Perhaps I should have said "S1 to S1b" to make it a bit clearer. Big Engine vs. Niagara. A few nominal DBHP down but better working adhesion at most speeds for the 4-8-4. Also the advantages of Timken would be seen over the older style of lightweight motion.
Additional fun if you modified a RailDriver or similar platform to have proportional controls and effort for each of these locomotives, and simulated 'typical' or exemplary PRR and NYC enginemen handling the acceleration to speed. (Do the passenger cars have prototypical engagement of the undercar Spicer generators? if not, that's a good thing to implement...)
Pity the comparison of T1 to S2a wouldn't tell us much about Franklin type A...
I also hope that there was an official testing report focus on the differences of performance between PRR S1 and NYCentral S1b!
RailDriver is an interesting device and good for the elderly since a user doesn't need to adjust the throttle with a little mouse and arrow! But you are right that some modification will have to be done for steam locomotive's simulation. 
. These train simulators available on the market "probably" can't simulate the differences of performance between an engine using poppet valve gear or conventional gear, a user can only adjust the parameter manually base on estimation or assumption.
A mix and match :-P
In the game, I didn't edit the engine config file of the Niagara, but their overall performance is as durable as N&W Class J but I might give it a try to see if the data in the config file of Niagara in the game can be further improved for better realism! There is no undercar Spicer generator in this game and details like valve travel in the cylinder: the lap, the lead, the exhaust clearance, valve diameter seems to be not applicable to improve the realisticity...But I can make a car with your avatar on it within 5 mins. :P
In the forthcoming video, I will try to demonstrate S1's wheel slip problem if the engineer starting up the train at maximum cutoff (70.6? %), apply full throttle. Using updated data, PRR S1 will only start moving if the cutoff was set at 20% or less plus full throttle. Within 12 mins, it will reach 100mph+ with 1360 tons behind her, depends on the gradient of the track (I am not using a level track). I don't know if this is realistic or a bug or glitch of it but I can't even start the train on level track at 30% cutoff due to wheel slipping. Once the engine started accelerating, she will run like a wild horse (with 1360 tons behind her!!) Was starting a steam locomotive at very short cutoff a normal thing in the past? Thanks a lot!
-Further reading-
In this thread on Trains Forum:
"An assessment of the benefits of the application of Franklin valves on the
PRR K4 and T1 classes"
Link: http://cs.trains.com/trn/f/740/t/272776.aspx
Forum member Dreyfusshudson (He has very rich experience in railroading, including developing and using computer software to simulate different steam trains performance, but he said he prefer to stay anonymous in the forum) showed us a table in one of his post:
Dreyfusshudson
Thanks, Jones 1945.
As tim z points out below, it’s not quite as you think- the resistance of the locomotive would not change, only the coach resistance would fall in proportion to the weight, and the figures themselves are not directly useful.
I have two answers to your question about how long it would take 6100 to accelerate 1065 tons to 100mph on level track.
The first is, if we assume that flat out working is 7200HP (I don’t know how this reported figure was arrived at), and that when starting you can put down about 70% of the rated TE without slipping (Pure guess, about the maximum normally applied ion the UK), then you reach 100mph in 11 miles, about 9¾ minutes. After that, you can maintain 100mph with about 5400HP. During the acceleration, steam rate is in excess of 100000lbs/hr, and you are burning about 17000lb/hr top quality coal. This kind of time is what you might claim for PR purposes.
However, my sense of the way locomotives were actually expected to perform is somewhat different. Starting is about TE, sustained performance is about how hard you are prepared to steam the boiler. As far as I can tell, it was unusual for US boilers to be steamed much above about 600lbs steam/sqft/ grate/hr, sometimes less. (This figure refers to top quality Coal of about 13500BtU/lb; it would be less for lower calorific values). Above this specific evaporation rate, unburned coal losses rise sharply, and it may be that this was a prudent level to avoid high boiler maintenance costs, e.g. due to cinder cutting, by not asking too much of them- the draughting usually allowed up to at least 900lbs/sqft/hr- a whopping 120000lbs/hr for the S1.
Looked at this way, the Table below shows how a number of different locomotives might be expected to perform with a 1065 ton train on a level track, again working at 70% maximum TE up to 20mph, maximum steam rate about 600lbs/sqft/hr beyond that. All numbers are just indicative of relative potential, not precise.
|
Speed after 20 miles and elapsed time
|
|
| |
final speed, mph
|
elapsed time, mins
|
sustained HP
|
|
PRR S1
|
96.8
|
16.50
|
5500
|
|
NYC J3a
|
84.6
|
18.75
|
3750
|
|
Niagara
|
90.4
|
17.25
|
4500
|
|
T1 Franklin
|
86.9
|
18.00
|
4100
|
|
T1Walschaerts
|
86.2
|
18.00
|
4100
|
|
K4
|
71.2
|
24.50
|
2400
|
Not even the S1 can reach 100mph, though all are still accelerating slowly, and the S1 would eventually get there. The Niagara comes out better than the T1 versions, because the PRR banked the supposed engine efficiency advantages of the Franklin valve and reduced their grate size from 100 to 92 sqft, thus, according to my criteria, reducing the maximum sensible steam rate.
Of course, on any real line there will be gradient fluctuations, which might enhance acceleration. On the Water Level Route and PRR Crestline to Chicago the gradients are gentle, but the eight coupled designs would all get up to 100mph after a while, and cruise in the low 90s. And, if a train is overloaded or running late, there is likely plenty of upward power reserve to knock a hole in these figures...
If our forum members interested in this topic, please read the original post for complete messages! I believe there is no doubt that the PRR S1, a train designed for speed can surpass the design top speed of Niagara, N&W Class J etc.
Anyway, all these simulations can be seen as a reference since there are way too many things that general computer cannot simulate. 
But the result about PRR S1 probably explained why she was assigned to the Trail Blazer, a money tree of PRR before the decline started in 1946. Note that there were a few stops before the train arrives Chicago or Crestline, Ohio, which required an engine had a reasonable acceleration rate and ability to reach higher speed to stay on the schedule.
"Why can't we be friends?"
The color of the Keystone plate will be updated.
By the way, BaltACD, Thanks for the song! 