I found a bug : the capacity of a train car seems to depend on the order the passengers are added. In this image, the leftmost 3 passengers can be moved around freely within the car, but the rightmost 3 passengers will get the red outline when picked up, and be ejected to the station platform. To put everyone back in the car, I have to remove one of the smaller passengers and re-add them last.
I didn't use the item that makes them fatter; this situation just arose naturally.