Hi everyone. I'm still finalizing my HO track plan & there's a spot where a bridge will carry one track over another. I wish to minimize the vertical separation to avoid steep grades in the vicinity, since the tracks have to meet up again. Right now I've got it down to just over 4" railhead to railhead at the crossing point. I'll use whatever kind of bridge works best. Should 4" be enough? Are there special bridges for this?