Because CBTC is a highly advanced system used for trains to run closer together and needs to consistently be tested in order for them to work safely for when it is active on a line/s, we won’t have a incident when God Forbid two trains collide because of a faulty computer system. Plus the process is long as some branches have to shut down entirely for certain weekends to not only replace old 50 year old signals, but to do cables and all that technological stuff which takes a while and cost money. Plus you add delays while testing trains along that corridor. I wished the MTA would do ATC systems like most other transit system uses like CTA, WMATA, BART and etc but it’s too late now.