Wow! Now you're talking!
I was going to suggest the Nitinol wire as an actuator solution. Nice to see that you are ahead of me on that.
Regarding the air hose connection I was thinking that the metal coupler would be adequate. It of course requires that a wire be connected to it somehow which is a little issue. There is the possibility of combining the two connection ideas such that a return circuit is available. This would negate track pickups in each car which is huge.
Also since I published this idea I realized that a simple pulse train could be improved on. Instead send a train of pulses synchronized with the DCC signal as a multibit message containing a header, a byte representing the car count, followed by a CRC. This could be sent many times for redundancy.
Processing in each car could be handled by a small microcontroller such as an ATtiny85.
The idea of triggering sounds from the cars is brilliant! Hearing a coupling sound coming from the loco when the actual action was 5 cars back forced me to turn off the sound.
Pretty sure you are on the right track so to speak. Carry on.

George
Edgewood WA