This game's biggest issue is that it lacks a really good feedback loop that makes you have to keep expanding and upgrading your infrastructure. I know we have the demand modifier but having a blanket increase across the entire neetwork feels half done, plus some cities, no matter how high the modifier is, won't fill trains due to their population densities (cough cough america).
My suggestion is that the rate is higher the more trains serve a station, and you can set how strong or weak this growth rate can be so you can have it be more relaxed or not. This would also make playing with money fun as you can see returns on your investment for transit from the population growth along the line.