Originally Posted by stut

Well, there's a few things you can do:

1) Lengthen the trains. Mostly not feasible as they run the full platform length in Paris anyway, and the stations are already very close together.

2) Improve the signalling to decrease the headway where possible. Having said that, 30tph is already close to current safety/technology limits. IIRC, the newest London lines manage 33tph.

3) Increase the number of walkthrough trains (like on lines 4 and 14), which increases the amount of space inside. Taking London as an example again, this has been revolutionary for the sub-surface (Met, H&C, Circle) lines.

4) Provide reasonable alternatives above ground, be it new lines, RER lines, Vélib, trams, improved buses, or ways to bypass the busy central area.

5) Provide variable pricing throughout the day to discourage non-essential journeys at peak time.

6) Provide information to the general public as to which are the busier and which are the quieter sections, and when.

Sounds like RATP have been looking at some of these, particularly 3 and 4. But I guess there's only so much you can do with the funding you have. I don't know what level of crowding you're talking about, but when I used to commute from Clapham Common in London, you would regularly have to wait for 5 or 6 trains to pass before you could physically fit inside - now that's overcrowding...