train clearance height

Are you running traditional O or scale? Also, what kind of track? Something with roadbed like fastrack places the trains a tad higher than tubular would

5" will allow for for just about anything O gauge with the exception of an elevated crane car.  You could go lower, but you'd never be able to add anything taller later.

I used 8" so I could get my hands between the top of the cars and the level above.  Less will work, but try to think through all the issues.

Art

I'd recommend 5 1/2" clearance.  If you look at the Lionel Auto Carriers and the 86' boxcars, they're very close at 5".  We had a bridge on the club layout that was 4.8" and the auto carrier scraped on the way under it.  Also, the clearance is stated from the tops of the rails, not from the platform.  You need to allow for the roadbed and track height in addition to compute the total clearance for a raw tunnel. 

Besides, my Wienermobile needs 5", so if I ever come to run, I'd like to make it all the way around.

What these guys are saying is pretty much right, but if you run electrics like a GG1 with pantographs up, you'll need even more clearance. The 8" is probably the best answer

Yep, I have a few electrics with the automatic pantographs, I have to remember to latch them down  when running on the club layout.

SoxFan,

    I like to leave 10" between my different multi-levels, this accommodates even my big Tin Plate 810 Crane Car Boom, in a decent upright position.  

PCRR/Dave