I think modder named: 'kenny456' on modhub made a mod for towing chain where you can attach multiple vehicles and all of them contribute to pulling power. Maybe he can help in figuring out how to do that if you ask him.
Viewing post in Placeable Railroad Track comments
yes I have been experimenting with the towing chain, I was able to get it to connect 2 locomotives and have them both pull, butt it was somehow less power than just a single locomotive.. I’m sick right now so I’m not getting a whole lot done, butt at some point I’ll get back to experimenting with this. Maybe I’ll reach out to him also like you said!