The “impending collapse” of the Gospel Oak to Barking Overground line has been averted and passengers will get a free month’s travel once the delayed new trains finally arrive.

Transport chiefs announced today that three Class 378 trains will be borrowed from other Overground routes and used on the problem-ravaged line. The first will run from Monday and the other two will be brought in before March.

The four-carriage trains will be more than double the size of the existing diesel ones and will help ease the pressure on the line.

Its existing Class 172 trains are slowly being removed as the lease is up on them but there is still no sign of their replacements, the electric Class 710s, which are now a year late owing to a farcical series of gaffes.

Last week Barking to Gospel Oak Rail User Group (BGORUG) secretary Glenn Wallis hit out at TfL after issuing a “crisis bulletin” warning the service would grind to a halt in “days” if no solution was found.

The borrowed trains will run alongside the remaining diesel ones, but by the middle of March the last of them will have been transferred to West Midlands Trains.

If the new fleet isn’t ready by then the service will be down to just the three borrowed Class 378s. Transport for London (TfL) says that would mean a reduced timetable, which will be “thoroughly communicated” to passengers in advance.

In welcome news, Sadiq Khan has also held talks with manufacturer Bombardier Transportation and secured a free month of travel for passengers once the new trains are eventually up and running.

TfL’s rail chief Jon Fox apologised and said he, Mr Khan and deputy mayor for transport Heidi Alexander were pushing Bombardier to get the trains ready.

He added: “We had expected the new trains to be in service well before now and are doing all we can to minimise impact on our customers who have been waiting for way too long for the trains they have been promised.

“To show our appreciation the mayor has secured funding from Bombardier to support a month’s free travel on the line once the new trains are fully introduced. More detail will be available closer to the time.”

The Class 710s were meant to have been introduced in January last year but problems with the software have delayed that massively. Once they are ready, drivers will need to be trained.

London Assembly Member and Islington councillor Caroline Russell (Green), who chairs the Assembly’s environment committee, said: “Free travel later is welcome, but these vital rail upgrades are repeatedly proving more complicated in practice than in theory.

“TfL needs to learn from their delayed projects and build in scrutiny and time to troubleshoot so they are not caught out again, without trains to run.

“The trains TfL have found to keep a service on the Gospel Oak to Barking line are the same ones used to maintain reliability elsewhere on the Overground network.

“There’s a worrying risk to the resilience of the wider Overground service until the new trains arrive. Passengers already see regular ‘minor delays’ from faulty trains. Allocating trains to the Goblin line will reduce TfL’s ability to keep these delays to a minimum.”