[Arnt]

Your suspicion is correct, under conditions that generally apply.

Trains on level ground have extremely low rolling resistance. This means that if you have enough train sets for peak service, running the train sets all day often makes sense, because the additional cost (in terms of CO₂) of operating a train set for an additional hour is so low. It also means that adding passengers to a half-full train is practically free (again, in CO₂).