Leading Assembly members have raised concerns over Sadiq Khan’s headline-grabbing plan for Transport for London (TfL) to take over key routes and stations from Network Rail in London.

Last month the London mayor said responsibility and funding for key rail assets such as tracks and stations should be transferred to the transport body to plug the communication gap between the rail operators and Network Rail, the current provider of the infrastructure.

He said devolving responsibility to TfL would enable it to run a metro-like service in and out of London where trains would run as frequently as the Tube.

However, questions have been raised as to how TfL, which is straddling a deficit of £500m – reduced from £1bn – would be able to take on Network Rail’s assets, when the railway owner’s debt amounts to £50bn.

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It has emerged that TfL has yet to carry out any assessments of the liabilities its stretched balance sheet would have to absorb as a result of any potential policy.

Keith Prince, the Conservatives’ transport spokesperson in the London Assembly, told City A.M: “Once again, Sadiq hasn’t done his homework. With a £50bn debt, taking on Network Rail’s assets would be a great burden. Sadiq is good at creating debt so perhaps he wants to improve on his record of putting TfL into a £1bn deficit by taking on another £50bn.”

He added: “In principle, the Conservatives are not opposed to TfL being responsible for metro services in London, but on the mayor’s record there is no way we could agree to him being in charge of anything.”

Caroline Pidgeon, chair of the assembly’s transport committee, said that while she was in favour of any London mayor having control over the suburban train services, full control of rail infrastructure may not be necessary.

“Instead of constantly talking about long-term changes the current mayor needs to concentrate on getting the basics right and ensuring he uses his existing powers to full effect,” she said.

“A Boxing Day service could for example operate on the London Overground this year – that is entirely down to the mayor of London. Getting to grips with the fiasco of the Barking to Gospel Oak line should also be at the very top of Sadiq Khan’s in-tray.”

“The mayor should stop constantly making excuses and take charge of what he is actually responsible for.”

A spokesperson for Khan said: “The mayor believes that in the long-term devolving London’s suburban rail services and infrastructure to TfL will improve the unreliable and dysfunctional service currently provided by the private rail companies and unlock existing, unused capacity.

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“TfL’s recently published strategic case for metroisation sets out our ambition for the coming decades – it is not a document about specific funding arrangements.

“Ultimately, the current system is broken beyond repair and there is a chronic lack of coordination between the rail operators and the providers of the infrastructure.”