I've easily carried 6 bags on a train — 1 backpack, 2 bags full of stuff, 2 suitcases and a shoulder bag with my papers in it. It's not that hard. 3 Gym bags is easy — 1 over each shoulder and carrying one, or 2 over the shoulders and carry one.
Also, some people have to take a lot of stuff because of external circumstances; perhaps they're bringing snow equipment or are trying to comply with weight allowances. In the case where I was taking 6 bags, I was transporting 3 bags worth of clothing, old toys, gifts, and a bunch of other stuff for my aunt. Since I was staying there for a significant amount of time, I required 2 bags of personal gear (clothes, electronics, and sports gear), plus an additional one for food for the journey.
3 bags of clothing when you're moving elsewhere (moving back from uni, or moving to a Co-op site) isn't that crazy.