the problem with this is changing from one machine to another turns the dc on
and off way too much. you want 6 or less starts per hour for good motor wear.
more generated way to much heat.

Hmmm... in addition to wooddorking, I also mess around with electronic
junk... I especially love microcontrollers... I bet I could build a small
uC board that takes switches from each gate like that, and keeps the dc on
for awhile after the gate closes, so if you open another gate it never
shuts off... probably could do it with a 555 also, but the uC would let me
add features (if I could think of any)

Also like to figure out some fancy solenoid system for each gate, so that
all gates are locked closed unless the machine in question is turned on...
We have over a dozen gates, and a lot of them are in inconvenient places,
and we have three DCs. I *really* want a more automated system :P