I have figured out what is wrong with my brain! On the left side nothing works right, and on the right side there is nothing left!
If I remember correctly in the old roofwalk era the AAR required that any top-of-rail to underside-of-overhead-structure less than 22 feet be marked with telltales to keep employees atop the cars from creasing their noggins. These close overhead clearances caused some railroads - predominantly in the east I do believe - to print warnings in timetables prohibiting employees atop cars on certain sections of track; this is also one reason why dome cars were relatively rare east of the Mississippi River. The AAR recommended a top-of-rail to top-of-rail separation of 26 feet. Before the Susie-Q could convey double stacks to Little Ferry(?), New Joisey it had to expend considerable monies lowering the floors on several tunnels. This was also a prime motivation on why Southern/Norfolk Southern daylighted "The Rathole".
How much clearance you are going to need is going to be determined by what kind of equipment you are going to run under it. In most cases this is going to be determined by the nature - read: heighth - of the locomotives you are using but that, of course, doesn't hold true in the case of autoracks or double stacks. My N-Scale standard is 2 inches model/26 feet eight inches prototype. Since about the only time these days that you are likely to encounter personnel atop cars is in Hollywood's B-movies I'm not even sure that telltales are still in use; my Seaboard and Western Virginia Railway no longer uses them. I do run autoracks and double stacks and I use either through-plate-girder or through-truss bridges.
The bridge separating your tracks could be one of those through-plate-girder, or through-truss bridges but your railroad could - and probably would - have erected some kind of deck structure spanning your creek as those were usually cheaper to build.
From the far, far reaches of the wild, wild west I am: rtpoteet
Modeling the fictional B&M Dowe, NH branch in the early 50's.
One quarter inch taller than you tallest piece of equipment, plus track and roadbed height.
Seriously though, you should get an N scale NMRA gauge and use that, plus track and roadbed height. Just because you say now that you will be runnning "small" equipment doesn't mean that you won't change your mind.
Elmer.
The above is my opinion, from an active and experienced Model Railroader in N scale and HO since 1961.
(Modeling Freelance, Eastern US, HO scale, in 1962, with NCE DCC for locomotive control and a stand alone LocoNet for block detection and signals.) http://waynes-trains.com/ at home, and N scale at the Club.
Another question for the N scalers here... as part of my always in progress track plan, I will be having a bridge crossing over a section of track and a small stream. The grade on either end will be 2%. My layout's setting is a B&M branch line in central NH during 1952, therefore I won't really be running any tall equipment. What is lowest height for bridge clearance I could get away with while still looking realistic?
Jason