Raised on the Erie Lackawanna Mainline- Supt. of the Black River Transfer & Terminal R.R.
if you mean you have 3 inches above the top of the rail to the lowest point of the overhead then you have a scale 21'-9" of clearance. i don't believe there was any equipment in that era that tall. much later when the tri-level auto racks came along 19' was the tallest i ever saw.
grizlump
I think he might be talking about 3" railhead-to-railhead, considering the design on which he has based his layout. So the actual clearance would be less once one considers height of the rail, tie thickness, roadbed (if any) and subroadbed.
For a short overhead crossings at 30 to 90 degrees or so to the line below, very thin subroadbed can be used above to help clearances. For long areas of overlap, the challenge is greater. The NMRA gauge is probably adequate for accurately-scaled '50s-era clearances.
There are a few items of model equipment that are not perfectly scaled, particularly in ride height (sometimes resulting in cars being too high over the rails). Some of the worst offenders seem to be older models of intermodal cars with containers and auto carriers (not an issue here) and unusual equipment like MOW cranes or tall flatcar loads, as mentioned. There has been the odd steamer that's larger than "AAR Plate B", but they are rare these days, I think.
The only way to know for sure is to check rolling stock that seems higher than normal. Accurately measure the real clearance you end up with and and make up a simple "go/no-go" gauge out of a piece of carboard or styrene mounted on a scrap of plywood with some track. Then check the pieces of equipment that seem "high" before operating.
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If you have 3" clearance railhead to subroadbed above, obviously you're in better shape than railhead-to-railhead. Good luck with it.
in case you haven't figured it out yet, just multiply your actual measurement by 87 (actually 87.1) to convert any dimension to scale inches and then divide by 12 for feet.
3 x 87 = 261 261/12 = 21 with a remainder of 9. hence 21'9".
Eastern roads tended to have tighter clearances than western roads, so if you are modeling the Lackawanna, as your signature suggests, you might be able to squeeze it down some more. The Erie had very generous clearances, and the NYC, NH, and PRR tended to be the tightest of all, in order to fit through the tunnels leading in and out of New York City.
About 15' over the railhead will accomodate lots of 1950, eastern equipment; that scales to 2.07".
An Accurail tri level auto carrier WITH a load of vans on top is 2 and 7/8" tall. A 40' hi cube box car is about 2 and 5/16" tall. (measured from the rail head.)Hope that gives you a frame of refference.
Do you need to allow for some additional clearance in case you have to get your hand in the space (to retrieve or re-rail a derailed car? Might be a good buffer to add.
The question was asked about steam era, 1940's, railroad undeclared.
Looking through my PRR diagram books, I'm finding 13'-6" for passenger equipment, 14'-11" for a Lines West caboose, and 15'-2" for a big auto box car. A 250 ton wrecking crane is 15'-6". The 1940's was the end of the short car era, so lower away.
15'-6" scales to 2.14" in HO. Add a quarter inch for safety and you get 2.39".