Though I don't know all the details, each new rail transit vehicle usually represents advances in technology, with more and more equipment managed by onboard computers. And, of course, each manufacturer has its own software. So, getting equipment from different manufacturers to work together is no small feat.

The following article that I did for Mass Transit magazine may help shed some light on some of the issues:

Rail modernization in Mass Transit magazine

Manufacturers can usually get older and newer models of their own equipment to work together, sometimes by updating software or controls on the older equipment -- but in this case, they have full access to all the design and sofware on both vehicles.

-- Ernest