For reference it would be good to have per-passenger numbers for "sitting on a diesel bus", "sitting on an electric bus", "sitting on a tram", "sitting on a commuter train" as well.

The marginal cost of one extra passenger is going to be very nearly zero. The vast majority of the cost is just moving the bus / train / plane, and the overhead / inefficiencies in the system. I've seen somewhere the numbers for one passenger on trains and planes but I can't remember where that was. Just know it is a very very small amount for the added weight of one more passenger.