Length is limited by platform length and width is limited by tunnel loading gauge and platform sizes. To increase the platform length, you have to do it at most if not all stations. Crowded stations are usually in desirable areas which make it harder to dig or acquire land.
> run more trains
For lines at capacity, I believe it's usually limited by trains dwell time. Longer and bigger trains take longer for people to safely board. To improve this on the train side, you can have more doors and bigger doors. Station platforms also need to be bigger, have more stairs, bigger walkways, etc. Longer trains also make it tougher for train drivers and station staff to open and close the doors safely.
Tough but not impossible problems but many solutions contain trade offs. Only surefire way is to build another line but that costs tons of money.
Length is limited by platform length and width is limited by tunnel loading gauge and platform sizes. To increase the platform length, you have to do it at most if not all stations. Crowded stations are usually in desirable areas which make it harder to dig or acquire land.
> run more trains
For lines at capacity, I believe it's usually limited by trains dwell time. Longer and bigger trains take longer for people to safely board. To improve this on the train side, you can have more doors and bigger doors. Station platforms also need to be bigger, have more stairs, bigger walkways, etc. Longer trains also make it tougher for train drivers and station staff to open and close the doors safely.
Tough but not impossible problems but many solutions contain trade offs. Only surefire way is to build another line but that costs tons of money.