Luton station is on the Thameslink line
The BBC has learnt that funding for the Thameslink line upgrade is "still in the balance".
The £5.5 billion project is meant to deliver longer and more frequent trains on the Bedford to Brighton route.
A funding decision is expected by the Department of Transport in the next few weeks as part of the National Infrastructure plan.
Phil Marsh from Railway Magazine told BBC Three Counties Radio how important the Thameslink upgrade is.
"It's absolutely vital for people in London and the South East" he said, "because it takes people off the underground and off buses when getting across London.
"It means you can just get on one train to the centre of London whether you're living in Brighton, Bedford or London."
A lot of the upgrade has already been done, including the work to increase the length of the platforms which will tackle the overcrowding problem that underpins the whole project.
"At the moment you're seeing the physical side of the work which is the platform extensions that will allow 12 coach trains to operate" said Mr Marsh.
"The current maximum is eight coaches, so people can see that that represents a 50 per cent increase in available seats.
"What you can't see going on" he added, "is the signalling upgrade which will allow an increase of trains from maybe 12 trains an hour through London to hopefully 24 trains."
However, with fares set to rise above inflation and many jobs in doubt, Mr Marsh agreed that the predicted rise of passenger growth may not come to fruition.
"The funding is in doubt" he said, "and if the new fleet of trains is in doubt, that means they won't be able to run 12 coach trains, and they also won't be able to run the intensity of the planned service.
"But of course with a lot of people going on the dole - to use an old expression - and fares rising well above inflation it's a very neat political decision which gets rid of the capacity problem for a few years."
Plans for a Thameslink upgrade have been around since the early 1990s in one form or another and Mr Marsh, who used to be on Railtrack's executive committee, explained that a cut to funding may not mean that work would just stop.
"I worked on the scheme at a senior level when it was Thameslink 2000 project" he said, "that was an £834 million project and this one is now £5 billion.
"The scope has increased and awful lot of work has been done, for example Blackfriars station which will be spanning the Thames and Farringdon which is coming on leaps and bounds.
"So they have to carry on with it, because probably like the aircraft carriers it's more expensive to stop now and you'll end up with half a railway line which is no use to anybody."