Very cool! I like the FET version as that would tidily replace my operating-boxcar circuit.
Would still like to see if this could drop-in to the MTH N6b operating caboose. The rub is the UCS method applies track voltage via 2 inputs - i.e., it's a double-pole controller switch. If the receiver only has a single-pole relay then if switching track AC into the receiver board, one side of track power must be pre-connected. This may or may not fly depending on the circuit design of the operating car.
The trick would be to see if the caboose can be activated by a FET output with common ground.
Is the plan to sell kits, just the link to OshPark bare PCBs, assembled/tested?