The thought is that "subway" mass transit can be full of people, even stuffed, but "commuter" lines are a bit lighter after they deboard passengers at the first few suburbs and you can find a nice comfy seat somewhere around the midway point.

Here in Southern Ontario (GO train) I have never seen aisles full with passengers. Although on the lower level there are some people standing, the upper deck is usually all seated. Often with some free seats.