The Gospel Oak to Barking Overground line will be halving its train frequency later this month – because of a lack of trains.

From March 18, three four-carriage Class 378 electric trains will run every 30 minutes on the route, which usually has a service every 15 minutes.

This is because the new Class 710 trains are more than a year late in being rolled out and the lease has ended on the existing two-carriage Class 172 diesel trains.

In the next two weeks they will all have been transferred to West Midlands Trains, leaving only the Class 378s – borrowed for this situation – to carry the thousands of passengers who use the line.

It is not quite the “impending collapse” as feared by the Barking to Gospel Oak Rail User Group (BGORUG), but it will have a major impact on commuters, even though the capacity of the Class 378 trains is double that of the Class 172s.

To make matters worse, there is no end in sight as manufacturers Bombardier are still fixing software problems on the Class 710s before drivers can be properly trained.

TfL’s rail director Jon Fox said: “We are very sorry for the continuing delay and share our customers’ frustration.

“Regrettably, despite our efforts, we need to release the last three diesel trains currently being used on the line.

“We strongly advise customers to check before they travel and re-time their journeys where possible to avoid disruption. We are providing detailed station-by-station travel advice on our website and station staff will have up-to-the-minute information to help customers plan their journeys.”

Electrification works for the line began in June 2016, when it was closed for eight months so overhead wiring could be installed. But the wires were “incorrectly designed” by Network Rail, and materials were delivered late.

That resulted in another two-month closure at the end of 2017 and it was hoped the new trains would be up-and-running in January last year.

Extra buses will be put on to help manage the problems, and regular passengers who choose to go via Zone 1 rather than brave the Overground will be automatically refunded within two weeks.

Mayor of London Sadiq Khan has also managed to get Bombardier to agree to a free month’s travel for customers on the line when the new trains are finally working.

Passengers can check out tfl.gov.uk/gospel-oak-barking-trains for information.