Oddly enough we're running Bombardier trains with an Alstom signalling system, and Ottawa is running Alstom trains with a Thales signalling system. Would things have gone better if we'd sole sourced Bombardier equipment and they had sole sourced Alstom? The software integration of disparate systems must be a nightmare, I'd imagine that each vendor does a fair share of finger pointing at the other which goes through several rounds before any actual problem gets solved. :-(
...K