I have done school trips with my dcs. They were younger than the children involved and I would never presumed that there would enough seats for all of them in the same carriage.
If the children had been 6yo and under then, I can see the dilemma for the teacher. I would have expected any child over 7yo to stand up in this case. And it would not have cross my mind to sue a teacher for that.
1- because a train breaking down to strongly that the child falls and hurt themselves is unlikely (or rather it is likely to be a serious thing)
2- because they might be moving around anyway (going to the loo anyone?)
I think that sort of attitude is H&S gone crazy. I can see the teacher would be anxious and have that sort of reaction but perhaps it is also to all of us (I mean all the one on here who say they would shocked to hear their precious dc had to stand up during a train trip of what one hour max and would be suing school if they hd got injured!!) to change pour attitude.
What we are talking about here isn't any more risky than what we do as parents and we wouldn't have an issue with it (I assume all of us would ask their dc to stand up for an elderly person here?). None of us is expecting to be getting sacked/going to a tribunal because we asked our dc to leave their seat for a disabled person.
It is actually scary because it means that, as a society, we think it's OK not to show any respect towards elderly people/'weaker' members of our society, all because of some sort of H&S gone over the top.