Passengers on the Gospel Oak to Barking Overground line will be offered a month’s free travel as compensation for delays to upgrading the service.

New electric trains – increasing capacity and reducing air pollution – were set to be introduced around a year ago.

But manufacturer Bombardier faced problems with on-board software which held up the rollout.

At one stage, the service was reduced to two trains an hour. But the line – which carries 30,000 people a day – has now returned to full operation, with trains every 15 minutes.

The new electric stock will double capacity, and also offer air-conditioning, USB charging, free WiFi, and more space for wheelchair users.

The Mayor has negotiated a month of free travel from August 31 to October 1 inclusive, funded by train manufacturer Bombardier.

Passengers will still need to tap in and out with their pay-as-you go Oyster, contactless card, or weekly or monthly travel pass – fees will be refunded automatically.

Those with annual travel cards will need to apply for a refund online, while paper ticket holders can recoup their fare through TfL’s customer services.

Deputy Mayor for Transport Heidi Alexander said a month’s free travel was “a well-deserved thank you to passengers” after delays in the upgrade.

Labour’s Florence Eshalomi, chair of the London Assembly transport committee, welcomed the replacement of “the old and antiquated diesel trains” previously running on the line.

She said: “After so many drawn-out delays, commuters who have been left to suffer unreliable services on the line will finally see some light at the end of the tunnel with this announcement.”