Firstly, lengthen trains (expensive to do in a subway as you have to dig the enlarged platforms ; it has been done on the Ligne 1 in Paris). Then, make a new, somehow parallel line, which is much more expensive but allows better service (as the line only has to double the most heavily trafficked part, and part ways later on : in Paris, see Ligne 1, Ligne 14 and RER A which are partly parallel between Gare de Lyon and Chatelet)

Un roi sans divertissement est un homme plein de misères