Keep a seat for someone later!
You will notice that when you enter a train, in most of the train cars there will be a reserved area for people who need the seat: these are called "priority seats". Of course, you can just leave it up to logic in regard to who actually needs the seat, but there will be some symbols on the window near the priority seats indicating the people who may need it: pregnant women, elderly people, injured people, people with small children, etc.
Occasionally, I see people who don't really fit into the above category, but they still really look like they could use a seat; sometimes people are literally dying on their feet from exhaustion.
Many people, both young and middle aged, will not respect the priority seat rule, and just sit down even though there are others who need the seat. That being said, it is sometimes a good idea to preemptively sit down in the priority seat, so that if anyone comes along you can give the seat to them. basically think of it as saving the seat in case someone comes. Better than letting some punk sit down who won't give the seat up!