IF I understood your project (it's a big IF!), I think you can also control the MTH Operating Reefer with minor re-wiring but no change in your Moteino code!
My $5 modification to an MTH Operating Reefer to make it remote-controllable is described here:
I understand the value of re-using the existing cables/connectors in your Signalman project. But in the case of the Operating Reefer, I chose to make it a 3-wire connection to the MTH electronics with no modification (cutting/jumpering a trace) to the MTH circuit board.
In the MTH Operating Reefer cars, +9V DC is always present on one of the motor terminals (Yellow wire). Circuit ground is always present on one of the micro-lever switch terminals (Red wire). And the motor drive trigger is the other motor terminal (White wire). To trigger the operating reefer to start its cycle, you simply "ground" the White wire for several seconds which is apparently what your single Moteino output can do via the SSR.