It was a highly complex integration of 3 different railway signalling protocols. These are safety critical systems that need to be "proven" to be safe through e

It was a highly complex integration of 3 different railway signalling protocols. These are safety critical systems that need to be "proven" to be safe through extensive simulation tests and real-world testing. The Crossrail/Siemens team that worked on this integration split the project up into 4 parts. 3 parts for each signalling system. And then the final part to allow transitioning a train between each signalling system in a seamless way. They were testing the trains on the western, central and eastern parts of the line for years as a way of gaining confidence in their work. It has been reported that this is the first time anywhere in the world that a railway has been created to support multiple signalling systems and be able to transition seamlessly between them during service.