You should consider buying a NMRA gauge. They are not expensive and they are very helpful in measuring clearances of overhangs, dock height, track geometry and several other things. You will find it indespensible if you have a problem with a derailment where the cause is not obvious. They are sold mail order and at most hobby shops that have railroad supplies. No I do not work for the NMRA but I am a member.
My overhangs are about 2 1/2" over the railhead so when I get cars knocked about by the tayranasauras-cat playing godzilla there is enough room to easily remove the cars. That happens very seldom, but a little extra clearance in welcomed. He sneeks in the railroad room every once in awhile. Sometimes I think he runs in when the door is open and then hides until I lock up for the night, then HE plays with the trains.
Paul
Dayton and Mad River Railroad
"No soup for you!" - Yev Kassem (from Seinfeld)