No less a vendor lock-in than DC-traction Kinki-Sharyo Type 7's from 1986 being retrofitted to MU with with AC-traction Breda Type 8's from 1998. There's no universal industry standard for MU'ing, but you can spec a compatibility baseline that accommodates a buyer's own mixed fleet. It's actually easier now than it was when the 7/8 MU'ing system was kludged together 20 years ago because the newest generations of rolling stock are so software-heavy that establishing trainlining compatibility is more about firmware programming than electrical/mechanical mods.Doesn't this lock them into CAF as the vendor for the Type 10s, though? I thought there was no standard for MU systems for light rail.
The break in backwards compatibility on the Type 9's was to ensure that whatever MU compatibility customization is done with the Type 10's stays largely firmware-oriented and requires fewest component touches.