Relay method described earlier:
If the $10 or so cost of external relay is too much, and one is comfortable tinkering within the electronics, I'm confident multiple ITADs can be linked. In other words there is one ITAD that is connected to the crossing gate(s). Additional ITADs would "talk" to this master ITAD telling the master to activate its relay when the remote ITAD detects a train.
I'd have to see the guts of an ITAD but I figure it should take less than 50 cents in components per additional ITAD to link it to the master ITAD.