Virgin Trains has lodged 400 pages of legal arguments as to why they believe the contract for the West Coast main line should not be awarded to FirstGroup, their rivals.
Since they lost the bid Sir Richard Branson, and many members of the public have built up a noisy campaign against the tender for the country's busiest and most profitable line.
But it is not clear that this last ditch legal challenge can actually stop it going ahead.
One lawyer told me that only an injunction could actually stop Justine Greening from signing the contract as early as midnight tonight.
A judicial review, which it seems is what Virgin Rail is going for may not enough on its own in a purely legal sense.
The question is whether it is enough additional pressure on the government to call a halt.
The Transport Secretary, Justine Greening, was adamant this morning she would not be changing her mind.
It would be unprecedented for the government to stop a deal after a very complex legal tender process.
But Sir Richard has told ITV News from Necker Island he believes the judicial review is enough to delay the deal.
He said it would be "completely against the understanding" they had for the deal not to be paused while the arguments are looked at again.
He said: "Judicial reviews are difficult to win but we feel we have a shot".
And he told me Virgin would run the line without profit while it gets sorted out.
Department for Transport sources tell me they will "vigorously defend" the process.
What is not yet clear though is if the pressure will stop the Transport Secretary from signing while that happens.