As long as the frame is not damaged beyond repair, a locomotive can be rebuilt/upgraded indefinatly. Here on the Union Pacific our switcher fleet is everything from SW1500/MP15, GP38-2 GP38-3 (Microprocessor controlled) and GP/SD40s and even some SD38s thown in for good measure. Even LIRR's 645 powered MP15s could be rebuilt with electronic fuel injection, microprocessor controlls, computer controlled braking. roller bearing traction motors, etc,etc. Or, MTA could buy some of these GP20Ds for switching service. At least these have a four stroke Clatterpillar.
how would you like to see these on commuter trains????????:-D