If you're going to have the physics right, you also have to change other settings as well. Since the road slug has no power of its own, the power it draws from the master unit(s) is not available to them to power their own traction motors. Also, on the Rock Island, the two road slugs used with U36B's cut out above a certain speed.
In my view, if you're going to do a road slug, it should be part of a set with a dedicated power unit, with both engine specs altered.
ns