Can't remember which tunnel in Washington State was recently refurbished after having been out of service for some time. To clear double stacks the arched roof had to be grooved to clear the upper corners of the top containers. Each groove was roughly triangular - top horizontal and side vertical. The photos I saw illustrated the result, but not how it was achieved.
Maybe someone else can enlarge on this.
As for protecting tunnel linings from steam exhaust, in the last years of steam JNR locos on mountain routes were fitted with big rounded blast deflectors. When entering a tunnel, a plate slid forward to cover the stack and clear a path for a fan-shaped, pressure-reduced blast backward across the whole the tunnel roof and along the train. The whole thing made the stack look like a mis-shaped mushroom.
Chuck (Modeling Central Japan in September, 1964 - in a Southern Nevada garage)
I use my NMRA Standards gauge as the minimum for the tunnel portals I've built for my railroad. I model Transition era.
NP 2626 "Northern Pacific, really terrific"
Northern Pacific Railway Historical Association: http://www.nprha.org/
CPR's standard was 22'-0" above top of rail, BUT in practice that was rarely applicable to tunnels. Concrete lined tunnels had an arched roof contour, since that was stronger structurally, and even rock tunnels tended to follow much the same shape, higher in the centre than the corners. While the corners could be lower than the 22 feet, the centre was almost always higher.
When double stacks came into general use clearances frequently had to be improved. The centre was not a problem, but that arch shape often meant the corners of the containers needed additional room (20'-10").
As others have noted, provision for overhead electrification probably required additional height.
John
On my last layout, I felt that the typical Woodland Scenics stone portals for single track were too high. So, I chopped the legs down a bit, leaving the apex of the arch, directly at the keystones, at about 3 1/4". All of my steamers passed under the bottom lip of the keystones with about 7/16" to spare. As Chuck warns, though, I had not planned to acquire a wrecker at the time, nor had I my fine Trueline CPR Caboose with its rather suprisingly tall stove stack.
Later, when I watched the refurbishment of CPR 2816 in 99/2000, and during the narration of its initial trip up the Fraser Canyon in south-central British Columbia, it was stated that they had concerns for the brand new sheeting, all nicely painted and polished, as the locomotive blasted up-grade through rock-lined tunnels that had not had a steam locomotive through them in 50 years. Fortunately, the blasting issue from the stack did not dislodge any rocks. But it got me to thinking that a tunnel servicing steam trains would want a certain clearance above the highest stack and most forceful blasts it was expected to accommodate.
Later still, I had occasion last summer to walk the Othello bridges and tunnels complex, all back-to-back, on the old Kettle Valley Rwy. With the roadbed scraped clear of any ballast and ties/rails, it is just a part of the Trans-Canada coast-to-coast hiking 'n biking Trail. Those tunnels are TALL!! I didn't estimate the distance between apex and where my feet were, but it had to be 21-23 feet. Adding the ballast and rails on ties, it would have been close to 18'.
What era are you trying to model, and what special conditions (oversize rolling stock) are you trying to allow for?
The quick and dirty (and, therefore, not-so-good) answer is three inches, and use the profile of the NMRA standards gauge - assuming that your models are HO scale.
That 15 foot tunnel height might have been valid for D&RGW narrow gauge steam, but it certainly wasn't valid for most roads. The USRA locos were built to that height so they could clear anything, anywhere - but even with that the tunnels had to be several inches taller. The tallest NYC loco was about 15'3" since it had to clear the Hudson Narrows tunnels. Take a look at a NYC Niagara to see what convulsions Alco Schenectady had to go through to squeeze that big barrel of boiler through those granite keyholes. OTOH, Southern Pacific and Santa Fe locos exceeded sixteen feet by several inches (and that didn't include the Santa Fe stack extensions.) Their tunnels were sized accordingly.
When measuring equipment to determine portal (and bridge) clearance, beware of such things as the `hump' in the boom of a wreck crane, or smoke jacks on cabooses. Of course, if you own a Schnabel car and run it loaded, all bets are off.
One special case in my immediate area - the five short tunnels (from the 1930s) on the ex-U.S. Government Railway ROW from the Lake Mead Visitors' Center to Hoover Dam. They were built to clear liner segments for the steel penstocks of the dam. The penstocks are 50 feet in diameter, and the tunnels are HUGE!
These are the latest double stack clearances. The first is for two standard containers, the second is for one high cube and one regeular container and the last is for two high cube containers. This information is from the CSX.
* Doublestack 1 — 18 ft 2 in (5.54 m) * Doublestack 2 — 19 ft 2 in (5.84 m) * Doublestack 3 — 20 ft 2 in (6.15 m)
Many -- not all, but many -- steam era tunnels built before 1900 accommodated lower equipment, but allowed for brakeman to work on top of the cars, since setting brakes often involved clubbing down brake wheels before air equipment was considered fully reliable. The tunnels that weren't able to clear men on top the cars had "tell-tales" to warn of low clearance coming up.
In the 20th century, there was a tendency to accommodate just the loading gauge of equipment. This allowed many older tunnels that had to clear men, too, to be adapted to taller loading gauges once the running boards weren't regularly used any more. It also saved on construction costs.Tell-tales went out of use, because work rules changed so that use of the running boards was prohibited and then they were eventually required to be removed.
So, you can do a variety of things with tunnel clearance beyond simply clearing the tallest thing you'll be running., depending on the situation and era you're modeling.
Mike Lehman
Urbana, IL
1st: This is an un-prototypical suggestion to take measurements from your larger/higher cars/engines, and; adjust the size of your modeled tunnel portals accordingly.
2nd: Another un-prototypical suggestion is to also adjust your tunnel portal size to adequate clearances for the sides of those larger/higher cars/engines.
3rd: Model the tunnel portals, whether purchased or custom-cast, to resemble tunnel portals of the prototype railroad and era you are modeling.
Note that nobody observing your layout will take time to measure tunnel portal dimensions with an NMRA scale ruler. It is ultimately the appearance of the tunnel portals that count.
See the Gallitzin Tunnels at North East Rails (between Johnstown and Horseshoe Curve) with the Pennsy's reconstruction of the Gallitzin Tunnels -- This is how the prototype did it.
Conemaugh Road & Traction circa 1956