RAIL union TSSA has called for a “proper explanation” over what caused a rare “do not travel” alert from Britain’s largest railway franchise during today’s morning rush hour.

Train operators Southern, Thameslink and Gatwick Express — all part of the Govia Thameslink Railway (GTR) franchise in south-east England — urged passengers not to travel on Wednesday morning “if at all possible” because of multiple incidents.

A train derailed near a depot in Selhurst, south-east London and there was a fault with the signalling system between London Blackfriars and Norwood Junction.

The alert, which is normally used during severe weather, was issued at 7.52am and withdrawn by 10am, but passengers were being warned that services may still be cancelled, revised or delayed by up to an hour.

TSSA general secretary Maryam Eslamdoust said: “As ever our brilliant Network Rail members were at the heart of the operation to get services running again and they should be praised for their hard work in doing so. 

“However, the disruption has been extensive with a significant impact on very many people trying to get in and out of London during the rush-hour period.

“Our union looks at this through the lens of safety on the railways, that is and will always be the number one priority. So, it’s vital we not only learn lessons about what happened but have a proper explanation given the necessity to issue the do not travel warning.”  

Govia Thameslink Railway and Network Rail issued a joint statement apologising for the impact on journeys.