That data isn't Open Data yet. Some people, e.g. Tom Cairns at RealTimeTrains, have signed some kind of commercial agreement with TOCs to share the data, but I don't think this scales.
RDG also have a platform in the Industry Data Portal (IDP) called 'Connecting Train Identifiers' (CTI), but again, this isn't open and I gather access to it is rather difficult to get.
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