NYCT was, until this year, operating R32 trainsets still limping along from the 1960s that were repeatedly life-extended as newer (1970s) train types experienced structural failures & delivery of their planned replacement trains was repeatedly delayed. It's obviously possible to maintain old trains, but generally expensive & challenging, especially when they're still doing 100,000 miles a year, and things clearly slipped.

Especially because the trains were operating well past their planned lifespan because their replacements were ordered but not delivered, so the major overhaul that would that would normally be done to life-extend a train kept running for decades didn't happen.