What a bunch of vile people.
When person behaves like a bad child, treat them as one.
Next time just ask them questions,
Can you think of any reason other than pregnancy someone might need a seat?
What sort of disabilities would mean someone need a seat?
Why would someone not want to share their private medical history with a train full of people?
Are you aware of anything outside you bubble?
What the best way to deal with you when you are having a temper tantrums?
Do you normally get a good response when you shout?
Do you think some time out might help you think? Go stand in the corner and think about your behaviour?
Keep asking them questions, until they have had to think it through, do it in a patience parent/teacher voice. It helps the penny drop that they are being a brat, and have missed some information, which you are kindly helping them to realise.
Poor baby, it is not in for much of a start in life.