Take a look at this sign inside a British Thameslink commuter train. It tells riders which railcars are most crowded, so they can walk to a less crowded car.
The signs are on Thameslink's newest trains, and work via a "load weigh system" that estimates crowding based on the weight load inside each railcar.
Combined with Thameslink's open gangway layout that allows passengers to walk from car to car without leaving the train, the system increases the functional capacity of the whole train for very little cost. Far less cost than more trains or more tracks, certainly.
And like real-time train or bus arrival information, it's easy to imagine this sort of info being incorporated into smartphone apps. If that happens, riders would be able to pick a spot on the platform where they know the least crowded railcar will arrive.
This seems like a smart choice for any transit system facing crowding problems.