I wrote a rather lengthy (and probably faulty) response to your question, then inadvertently hit the big red X in the upper right hand corner of my screen and lost the whole thing. Sorry that I don't have time to re-write it all, but here is a link to what appears to be a former interlocking tower now in use as the office for the Vandalia RR. I presume the Vandalia is a "spin-off" shortline somewhere in Illinois.
http://www.rr-fallenflags.org/misc-v/vccr-off-acs.jpg
Hopefully, someone with more knowledge than I can direct you to other prototypical uses for tower structures on a shortline layout.
Good Luck,
Tom
Are these used today? Do interlocking signals still exist on short lines? I know, obviously, that interlocking signals have been "obsoleted" on all class ones by CTC systems today, but are these towers still used for other purposes, and for what purposes? Or are they basically all razed? Were there interlocking-type "towers" in yards anyway?...or is the idea of a yard tower obsolete too, are yard "towers" basically just low office buildings today?
I have a neat model of an interlocking tower that I'd like to use, within plausibility, for my shortline...looking for the excuse.
Thanks much, and have a great day...