Put an electronic sign up on each station platform indicating where the cars with most space will be on the next train. Some people will rush to those cars, some will just stay where they were. By varying the amount of notice given or the threshold where you consider a car "less full", you can distribute the travellers more evenly.

I suspect the reason this isn't valuable is that the feedback loop is already pretty good at high occupancy rates: when one car is at capacity, every other car is too.