Services which are overburdened can ask for taxpayers to meet part of the pounds 750,000 bill for every new carriage bought to meet the unexpected demand. Under rules set up when the companies were sold, owners can apply for more public cash to buy extra rolling stock if they are carrying more passengers than in 1989, when 813,000 people used trains in the South- east of England during morning rush-hour.
With some lines now seeing passenger numbers increasing by 20 per cent a year, many companies are expecting to run out of room on their services. Government figures last month showed five out the 10 train companies operating commuter routes serving London are already "too full". Connex South Central, which serves the south coast and London, has a "capacity limit" of 64,000. But the latest figures show 64,886 people using it in the rush- hour.
Growth of nearly 10 per cent in passenger numbers on Thameslink, which runs the Brighton-London-Bedford service, saw it carry nearly 2,000 more commuters in the morning peak than its 24,500 limit. John O' Brien, the Franchising Director, who monitors the network, says no operator has come looking for money yet to buy new carriages to cope with this "unforeseen growth".
But numbers are going up fast. South West Trains, which ferries commuters from Surrey to London, is only a few hundred passengers short of its 71,000 capacity limit and grew by 10 per cent last year. WAGN, which runs the busy Peterborough-to-London service, is only fractionally under its maximum load of 45,500 - having only carried 39,000 passengers in 1996.
There is no one explanation for the growth in passenger numbers.
Growth in the economy is one factor. But while London Underground has seen passenger numbers rise by 4 per cent, the "overground" rail companies have recorded a 10 per cent jump.
Some railway managers say housing developments on the edge of the city have seen a new "commuter class" develop. Martin Walter, a marketing manager with Thameslink, said: "We have seen new passengers from new housing in Harpenden [in Hertfordshire], where professionals want to get into the centre and with us it's only 25 minutes."
Others point to London's increasingly gridlocked roads. Civil servants at the Franchising Director's office say "congestion is now universal". "People no longer can drive to the edge of London and then take the Tube - because of congestion. So they take the train."
Executives also say the rail network may require an overhaul. "Personally, I think the infrastructure needs to be looked at first," said John Hampson, commercial director with Connex South Central. "There is no point in running more carriages on each train if the platforms in south London are not long enough to cope with them."