I use a drop section on my little round-the-room 9x16 attic layout which is still in a raw stage. The drop section has been recycled and used for the second time from my large dismantled layouts. The span length is 26" and width[depth] 20". Clearance is only 22" since I have added bridge rails in an effort to sort of disguise it as a thru plate girder bridge[20" is about the minimun walk-thru space I suggest].
It is hinged on a piano hinge and secures on the "strike side" by adjustable keepers, to maintain even rail tops, which receive the spring-loaded catches or "strikes"[components like on your household doors except adjustable to align rail tops]. The piano hinge holds the section very rigid which enables stable rail alignment side to side and for rail tops. Parts to build it, as Jim Barrett illustrates, are easily available.
My "Dropgate" is installed on an end curve, just inside the room entrance, supporting a dual 072/084 mainline. Hopefully I can get a photo up.
I suggest viewing Jim Barrett's "Backshop DVD #11", available from OGR,which illustrates how to build and install both "Dropgates"and Lift Bridges. [an o-gauge aquaintance in Arizona built his layout in a back yard construction trailer and installed both a drop section [lower level tracks] and lift bridge[upper level]. The 7' trailer ceiling was the "stop" for the lift on his bridge. His entrance door opened outward of course,]. A lift bridge is a very viable alternative if used in the straight section.
