The feed at:
Appears to be missing some key data about what lines serve which stations.
The element is empty for quite a few stations, for example (but not limited to) St Paul’s, Wimbledon, Kensal Green, the Heathrow terminals, and others.
Is there any reason for that / can this be updated and corrected?
The file you’ve referenced is a good source of information for station facilities, but the lines served information is not kept up to date - we would consider this one of our “legacy” feeds.
You should find that the Unified API has upto date information on Stations with their lines served, feeds, live arrivals etc. See Unified API Part 2: LoT – Location of Things – TfL Digital for a few examples.
The benefit to the Unified API is that it aggregates facilities information from multiple sources, including the one you have referenced. In future, we are looking to improve facilities information further (e.g. the accessibility of every bus stop in London) this will also be surfaced automatically in the Unified API.