CBTC has been the world's de facto metro signalling standard for 40 years. In all that time it has never been made interoperable across suppliers, so why is the question suddenly back on the table, and why is it proving so hard to answer?

My guest is Dr Alan Rumsey, one of the genuine authorities in this field. Alan chaired the IEEE working group that wrote the original CBTC standard, and he has spent a 45-year career on advanced train control projects across North America, Europe and Asia.

We start by getting our terms straight, because in this industry the same word too often means different things to different people. From there we work through the real questions. Why would a railway want interoperable CBTC at all? What does interoperability actually mean once you get specific about it? And if the business case stacks up, how could the industry ever get there? Along the way we look at why New York took 25 years and a fortune to achieve it, why ETCS managed what CBTC has not, and what China's interoperable standard might mean for the established suppliers.

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