Exactly, Look at whats happening with the 7000s in DC. The WMATA wanted to replace all of their older subway cars with one new car type but when the 7000s had to be pulled from service due to various issues, they retired so many older cars that there aren't enough cars to obtain normal levels of service, which make most lines operate every 12-20 minutes instead of every 6-12 minutes. That's why it's better to do so in batches so if issues arise and a particular fleet has to be pulled from service, it doesn't have catastrophic effects on service levels.