Obviously, for a given number of passengers, more space = more cars or larger cars. But I'm curious if giving passengers 8" of extra legroom is cheaper per passenger-mile for trains than it is for airlines. I'd assume so.
IIRC the NEC in America can accommodate 16-car trains but not all trains are that long.
IIRC in its early days JetBlue used to strip out a row of seats in its a320s to get a number divisible by 50. The rearranging of the rest of the seats allowed them to advertise having the largest domestic economy seat pitch in America, which was a happy side effect.
Planes also have higher energy consumption given that they have to move you vertically and horizontally, and electric trains have an even greater advantage since electricity is a lot less prone to swings than fuel.