In fact, deputy mayor of London for transport Val Shawcross, has said he'll be making "a last-ditch attempt" at getting the government to have a rethink.
"Although the decision has been now taken by the secretary of state, it's so important that our approach is that we're not going to just take no for an answer, it's too important," she said, speaking at a meeting of the Transport Committee at the London Assembly on Thursday morning.
"This is a cross-party agenda, it's about public services," Shawcross added, and said she was encouraging anyone who felt it was "a huge missed opportunity for south London and the areas around London" to add their voices.
She said the "drop dead date" for this round of franchising is likely mid-January, so over the next few weeks supporters of rail devolution in the capital will be doing "everything we can" to get the prime minister and chancellor to look at the matter.
It was revealed earlier this week that transport secretary Chris Grayling had rejected the London mayor's bid for TfL to take over Southeastern services from 2018. He told the Evening Standard that "the last thing our railways need in London is deckchair shifting" and said the business plan hadn't been convincing.
But a leaked letter from Grayling to then mayor Boris Johnson in 2013 revealed he wanted to "keep suburban rail services out of the clutches of any future Labour mayor". Grayling wasn't transport secretary at the time, but was justice secretary and MP for Epsom and Ewell.
Conservative MP Bob Neill has called for Grayling to resign over the leak. He said it was clear that "Grayling's decision was not motivated by the merits of the case but by party political bias".
For her part, Shawcross said the main thing to say was: "We're confident it was a very sound business case."
"We're hugely disappointed that the secretary of state hasn't taken this opportunity to improve quality of life and economy for south Londoners," she added.