It is not all that expensive to operate the old system while the new system is installed. They throw a burlap sack over the new signals until they are ready to

It is not all that expensive to operate the old system while the new system is installed. They throw a burlap sack over the new signals until they are ready to activate them, and otherwise the old system just remains in use and costs exactly what it would cost without the new system being installed. The old system has become somewhat more expensive because replacement parts are not needed anywhere else. Originally it was all standard railroad equipment and the same interlocking machines were used on other regional railroads (including freight lines). Since the MTA is among the last users of those machines, they have to fabricate some of the parts or special order others. That said, the old system is pretty reliable and does not require "constant maintenance" beyond what a railroad generally requires. Its shortcomings are mostly in its inability to run trains more closely together, its inability to identify which train is in which block, and its inflexibility with regard to track directions. Otherwise the maintenance is mostly replacing light bulbs and vacuum tubes, and rarely having to replace mechanical parts. In theory the lightbulbs and vacuum tubes could be replaced with solid state components, but in practice there would be no point -- you might as well just replace all of it with solid state equipment, which the MTA is (slowly) accomplishing.