If you seen a passenger on the opposite platform collapsing and banging their hit on the ground as they land, then further to that, take a siezure on the platform, would it be classed as tresspassing if you jumped across the track to get to their aid when there was no trains coming either way, however there was a train at a halt in the platform but going no where?
Rule Number One of First Aid: DO NOT PUT YOURSELF IN DANGER!
Otherwise the second person to arrive on the scene has to decide which of the two casualties is a priority. They might decide it is not you.
You never run to a casualty, you walk to them, assessing the situation as you approach, and exuding an air of calm authority. You only run if there is a fire.
But to answer your question, yes, it would be trespassing, and putting yourself in danger. If they have already hit their head, then they have done the damage, if they are fitting, unless you have an anticonvulsant in your bag, you are not going to get them out of it. If you want to be useful, use the time you are walking across the approved crossing to ring a grown-up on 999.