I try and plan the top of my train tables to be right about 40" or so. That always seems to be a good, all-around height for a layout. Taller than that may be okay for 24" wide modules or shelf layouts, but for a 4' or 5' or wider train table, I think much over 40" or 41" will start being a pain in the posterior for working on it.
I've gone in and sawed down legs in order to add casters and keep a train table at a 40" height.