How do we really know?

Well i'll take a stab at this.A steam engine is basically free wheeling when the throttle is closed(or at least so i've been told).Samurai will be able to answer this better than i can as he has fired on steam engines.Steam engines didn't have "MU" connections to be hooked up to diesels except for air.Most of the time you see a diesel behind a steam engine it's there in case the steam engine needs a bit of help on a grade and for it's dynamic brakes going down a grade.The diesel wouldn't need the help of the steam engine in the case of the engine brakes(or for main reservoir air either).We don't really use the engine brakes anyways til we actually come to almost a complete stop(new AC units have dynamic almost to a standstill).As for a free wheeling diesel the traction motors wouldn't be putting out any voltage.When a trailing unit is isolated the generator isn't doing anything.Without excitation to the traction motors they are doing nothing.