This is how it is today, but imagine you need to do pre-boarding to trains by TSA one hour in advance, increased density for seating so that premium-economy can

This is how it is today, but imagine you need to do pre-boarding to trains by TSA one hour in advance, increased density for seating so that premium-economy can be sold at a higher margin, baggage limitations and payment per kilogram, restrictions to wear a belt at all times (no moving around allowed). These restrictions were added to flying one by one, boiling the frog. It can and it may be implemented for trains also.