That may be the case for LA, I'm not familiar enough with their situation to speak to it, but it isn't the case generally. There are lots of rail lines in the US and globally that have not come anywhere close to maxing track capacity. One such example is BART in SF, which could just add more cars to existing trains and see 60% increase in capacity [1]. The tracks and stations support trains up to 10 cars long, but very few are actually that long enough because they don't have enough cars in the fleet.
[1] https://www.bart.gov/about/projects/cars/faq