I have commuted for years and years and years on tubes and trains (weeps gently at how much money/time I have spent on London transport).
Of course adults shouldn't give up their seat for a child. Unless the child is struggling. If it's a really busy train they won't 'fly' anywhere because you can barely move anyway. Or, here's an idea - try to plan your journey so you're not on a train at rush hour. Obviously if it's for a hospital appointment or something then you don't have much choice, but if it's just a day out at the Natural History Museum...
But generally of course you should give up a seat for anyone who might need it - pregnant, on crutches, looking woozy - I once gave my seat to a woman who was crying (I also gave her a tissue), generally a bit unstable.
Also, of course people shouldn't put bags on seats, or sit with their legs apart. Or, my pet hate, sit on the outside seat of a group and then expect you to clamber over when they could have just moved up.
And of course people can sit on a bank of six seats on their own. That's just mental. And actually, I'd go as far as to say, you shouldn't eat stinky McD's on a train either. It's anti-social. As is loud music through headphones.
And relax. Ooh I feel a bit better for getting that off my chest. Sorry!