You are right, I should make it easier to do that. Making other locomotives compatible with the system seems like a ridiculous amount of work as that would create a whole ton of compatability issues, however doing the same for a carriage should be relatively easy. Basically you just have to copy the microcontroller from the existing one and connect it the in same way. I will update the railcar to have proper labels soon.
Feel free to PM me you are working on something like this, i´d be glad to help :D

Great train, it is a shame the logic controllers outputs, channels etc aren't better labelled to make it easier to adapt other rolling stock to play nicely with it's data feeds

Nice!