Considering the fact that much of the same infrastructure they used in 1946 is still in use today (with 74 more years of wear and tear) its not surprising that its falling apart. This would be true even if the MTA wasn't staffed by dishonest, incompetent political cronies. When you regularly do things like spend $600 million dollars on trains that don't work, its not hard to see how you end up billions in debt very quickly.

https://www.nydailynews.com/new-york/ny-l-train-normal-servi...