Explanation
One thing that is a general quality of IR is that all propelling units are basically retextured locomotives, meaning that in the case of other players on a train, they can sabotage the train if they want to, causing delays and headaches. Mostly to solve this, my suggestion would be to add a type of unit, which is not controllable from the stock itself, but from a locomotive/driving trailer that can control it via multiple unit control. However, if the controlling locomotive gets detached, the unpowered trailer should be able to stop, so a runaway would be averted.
Potential Applications
-Multiple units with motorized cars not on the ends (Subway stock, modern tram stock, some multiple units with underfloor mounted engines, sandwich units, etc.)
-B-units

Potential benefits
-Less train sabotages
-More realistic multiple unit driving dynamics