Describe the suggestion
We recently were treated to the "multiple unit capable" feature for locomotives, which is a great baseline to start at, however, I think having a list of sorts for defining other stock the MUC stock is capable of interacting with would allow people to properly make stuff such as triplexs/autocoaches/control cars for specific pieces of stock, without it interacting with all other MUC stock.

Applications of the suggestion
Something such as having the options in the json be akin to:
"multiple_unit_capable": true;
"compatible stock": stock1, stock2, stock3, etc.;

Kind of rudimentary but an idea nonetheless.

Not a bad call, so just having the "compatible_stock" above be replaced with "connection_type:" and from there you could just have custom types if you make singularly compatible stock.

Like that a lot more actually.

It would probably be simpler to specify a connector type for the locomotive, and any locomotives that share the same connector type(s) could MU together. For example most modern North American diesels from both GE and EMD share the same 27-pin connector standardized by the AAR. Older diesels from different manufacturers used a wider variety of connectors, and many early electric locomotives were only equipped to MU with the same exact class.