Yes.
Of course they are able to stand.
But that's not really the point.
I don't know if you've noticed, but it's generally preferable to sit on a moving train, partly because it is more comfortable, especially over long distances, and partly because the movement of the train can make it more awkward to stand than it would be on solid ground.
(I believe the train companies are aware of this and that's why they have, for quite a long time now, put seats on trains.)
As a result, seats on trains can be quite a desired commodity. Even though, as you quite correctly have observed, grown humans are perfectly able to stand (unless they have a disability), most grown adults choose to sit down on the train when they can.
In those circumstances, and call me a snowflake if you like, I will try and do my best to make the journey more comfortable for my fellow grown humans. I will, for example, move my bag or coat off a seat so they can sit (even though they are, actually, perfectly able to stand).
And I will, if I have a 3 year old child with me, get them to sit on my lap. Even, yes, if they have paid for a ticket. Even though they are entitled to have a seat of their own. Yes, I will go out of my way to make someone else's day a little bit more comfortable even if it might make my own day a little bit less comfortable because that seems to me to be the sort of little act of kindness that makes the world a happier place.
But you do you.