Why does Chiltern train data go missing between Amersham and Harrow?

[Railguy1]

When I check RealTimeTrains and Citymapper, the data for the trains seem to go missing - indeed Citymapper often reports the train has already stopped at Harrow and on its way to London when this is not the case!

For example:

Realtime Trains | 2C29 1013 Aylesbury Vale Parkway to London Marylebone | 20/03/2026

Real-time train running information for 2C29 1013 departure from Aylesbury Vale Parkway to London Marylebone on 20/03/2026. From Realtime Trains, an independent source of train running info for Great Britain.

www.realtimetrains.co.uk

 

[Ian Hardy]

When I check RealTimeTrains and Citymapper, the data for the trains seem to go missing - indeed Citymapper often reports the train has already stopped at Harrow and on its way to London when this is not the case!

For example:

Realtime Trains | 2C29 1013 Aylesbury Vale Parkway to London Marylebone | 20/03/2026

Real-time train running information for 2C29 1013 departure from Aylesbury Vale Parkway to London Marylebone on 20/03/2026. From Realtime Trains, an independent source of train running info for Great Britain.

www.realtimetrains.co.uk

Because between Amersham and Harrow on the Hill the signalling is controlled by London Underground Limited and not Network Rail.

 

[centraluser]

Because the trains are running on London Underground tracks and using their signalling systems. The LU signalling systems are not fully linked / integrated with the Network Rail / National Rail systems, from which most other public facing systems get their information.

Some Chiltern services do show information for the Met Line stations at which they call. This is fed by GPS using geo-fencing, and needs a couple of technical systems to work together for it to work properly.