Mike, the road and the pedestrian walkway on the bridge look good!! I am glad you approve of my procedure! 
The AIU contacts can only handle 4 amps, the AIU terminals will energize the coil, or whatever you call it in these solid state relays, then the contacts will close completing the power circuit to the track, protecting the AIU from short circuits caused by derailments, etc. It sounds like you are thinking of the lack of the watchdog signal of the DCS engines starting up on energized tracks. I have a couple of devices I bought from Gunrunnerjohn to take care that. I don't even understand that whole electronics discussion, but a dummy can connect John's circuits and be in business.
I have never used these solid state relays before. When I worked for the power company, I was in the relay department. The relays detected faults on the transmission line, transformers, or breakers. Those were massive electro-mechanical devices. The relays in the power station protected the switchgear for station power, etc. Since the lines were so short, we would test the relays to trip at instantaneous currents of up to and over 100 Amps. 
Yes that's right; over 100 Amps. When we operated our test equipment, the coils in those relays would literally groan when current was applied! Those are the kind or relays I'm used to.
