Do you look at your seat before you sit down? And I mean either bus or train. It really doesn't matter.
Seats on both can get icky and hold boorish passengers who vomit or eat chicken dinners, and leave peanut shells on them.
These photos show various seat ickiness.
The last photo is a shot I took on a new Series 5000 rail car showing how two big guys hang over the empty seat(s) next to them.