Taxpayer faces £100m bill after collapse of Tube signalling contract The contract was scrapped after Bombardier, which is employing around 100 engineers in London on the project to upgrade the Circle, District, Hammersmith & City and Metropolitan lines, said its signalling system was incompatible with the Underground's creaking infrastructure, more than two years after the contract was first awarded. London Underground said the figures were an estimate and the lower figure could partly be explained by inflation since the original tender in 2008. But taxpayers could still be left with costs of at least £100 million given the sum that has already been paid to Bombardier. Bombardier is in line to win a lucrative £1 billion contract for Crossrail trains due to be awarded shortly, making the collapse of the Underground contract doubly embarrassing. Tony Travers, director of the Greater London Group at the London School of Economics said: "It is clear a lot of money will have been spent on a contract that has been terminated. The big question is who pays - Transport for London or Bombardier? Farepayers and the Government might also wonder why the new contract is so much more expensive than the previous one. He said sneaking the news out quietly between Christmas and the New Year was "worthy of [TV satire] The Thick Of It." He said the Department for Transport, which funds TfL's capital projects, should be questioned over the revelations. The project is forecast to increase capacity on the Circle and Hammersmith & City lines by 65pc, the District line by 24pc and the Metropolitan line by 27pc. Between them, the lines handle around 40pc of London Underground traffic.