Fears London's multi-billion Crossrail project may get too big to build have been raised in the House of Commons.
Plans to change legislation on the project so the line could be expanded are being debated by MPs.
But Transport Secretary Alistair Darling said the "hugely expensive" scheme must still be deliverable.
The debate is focusing on plans for the line to run from Reading in the west to Ebbsfleet in the east, rather than Maidenhead to Abbey Wood.
If this is to be possible, the remit of the Select Committee which will discuss the plans has to be expanded.
To do this, text of the Crossrail Bill must be changed.
Mr Darling told the Commons: "This is a huge project, it is hugely expensive, it will be a massive engineering enterprise and we must have a proposition that is actually deliverable."
Crossrail, which would ease the pressure on London's Tube network, was first proposed in 1989 and has been repeatedly delayed or shelved.
Tom Brake of the Lib Dems said the lack of "real action" in terms of building the line, was causing "disquiet" among the business community.
But Tory MP Chris Grayling said he did not believe the government had any intention of going ahead with the line.
He said: "I think this Bill and these motions are set to become just another chapter in the growing volume of government broken promises on transport."
The scheme faces opposition from people living near Brick Lane which will have parts of it dug up so tunnelling can take place.
Tower Hamlets Council said a survey showed there was no reason the tunnel could not be built from end to end.