They can be pretty tricky when they get bigger, so I can totally see how this can be frustrating. The trouble is that it also is part of what make the game challenging (and fun). It requires you to figure out new/better routes. If we took off the cap it would end up making the game easy (and boring).
That is a good point about Mumbai (and a few other maps). We can have a look at bringing up the train amounts on maps that only have 5 or 6 lines so they would be as so:
7 lines = 4 trains limit (28 total)
6 lines = 5 train limit (30 total)
5 lines = 6 train limit (30 total)
We'd definitely will want to test them out before committing to this change (be sure it doesn't make the game too easy). So no full commitments, but I can put it in the backlog and check it out in the near future.