Overcrowding on trains in England and Wales will get substantially worse over the next four years despite rises in ticket prices, a report by MPs says.
The Public Accounts Committee (PAC) said the Department for Transport's own plans suggested targets for increasing passenger places would be missed.
It blamed the failure on the absence of any requirement to improve capacity within train operators' contracts.
The government said plans to improve the situation would be unveiled soon.
Chairwoman of the PAC Margaret Hodge said MPs were concerned that the "already unacceptable levels of overcrowding will simply get worse and ever more intolerable".
Her report - Increasing Passenger Rail Capacity - said the fundamental problem was a lack of any incentive for the industry to supply extra capacity without additional taxpayer support.
Under their franchise agreements, train operators are required to use "reasonable endeavours" to give peak passengers "a reasonable expectation of a seat within 20 minutes of boarding", but there is no legal burden upon them to expand fleets or improve stations to achieve this.
Instead, the PAC said, it had fallen to the taxpayer to provide funds to Network Rail to carry out any upgrade work.
Over the past 10 years, rail passenger numbers have risen by about 40% and the industry is expecting demand to double over the next few decades.
The Department for Transport is 18 months into a five-year, £9bn investment programme to improve rail travel.
Under the plans longer platforms are being built and there will be more carriages on services into London and other major cities during peak hours.
But the Public Accounts Committee report says "this approach cannot go on indefinitely" and "alternatives must be found to meet the capacity challenge in the future".
It recommends that the DfT should require all new train carriages to be fitted with automatic passenger counting equipment to show how many people are travelling and when.
It also believes if more tickets were contained on smart cards, operators would be able to use the information about passenger numbers to target overcrowding.
The government should "pursue and promote smart ticketing and other demand management techniques to reduce the inefficiencies of overcrowding in peak hours and underused rolling stock at other times", the committee added.
But Mrs Hodge told the BBC that rail operating companies in the UK were more inefficient than those in other countries, so any reforms must be properly overseen.
"We don't think the regulator has been doing a particularly good job. We think it's time to look again at the way that Network Rail and the operating companies are accountable to the public," she said.
"[They] don't seem to think about how to use public money more efficiently. They think that the answer always lies in more taxpayers' money or in commuters paying higher fares."
According to the report, there will be 15% fewer extra places delivered in London in the morning peak than were promised by 2014. In other major cities, the shortfall will be 33%.
This compares with the numbers the DfT stated would be needed just to hold overcrowding at current levels.
The cross-party group of MPs highlighted parts of the South-eastern franchise, where passengers paid premium fares to support the new high-speed Javelin services "which do not stop at their stations and do little to alleviate overcrowding on the trains they use".
They added there was concern that the Office of Rail Regulation had been in place for more than a decade but "had not succeeded in getting a grip on the railway industry's efficiency".
A spokesman for the Association of Train Operating Companies said "sustained and targeted" investment was vital.
"Train companies have argued for longer, smarter franchises which would allow them to tackle overcrowding more quickly and cheaply by giving them more scope to innovate and bring in private sector investment," he said.
Anthony Smith, chief executive of rail customer watchdog Passenger Focus, said: "We welcome the importance this report places on getting a seat as it recognises the daily struggle faced by some passengers."
Bob Crow, general secretary of the RMT union, described the report as a "shocking indictment of the total failure of rail privatisation".
"Passengers are forced to pay through the nose to travel in obsolete and overcrowded carriages while private train operating companies are laughing all the way to the bank," he said.
Gerry Doherty, leader of the TSSA rail union, said: "Passengers face the worst of all possible worlds - ever-rising fares on an overcrowded service with no relief in sight."
He said the coalition's plans to raise fares over the next five years would "force many families off the railway".
In the government's Spending Review, it was announced that caps on regulated rail fares - essentially those within peak hours - would rise to 3% above the RPI rate of inflation for three years from 2012.
Responding to the Public Accounts Committee report, Transport Secretary Philip Hammond said the increases would enable investment in capacity to continue and details of the plans to reduce overcrowding would be unveiled in the coming weeks.
But he added: "We currently have one of the most expensive railways in the world, which is unfair on both farepayers and taxpayers, reflecting poor incentives to control costs across the whole industry.
"As the committee rightly says, this situation is not sustainable. We have to reduce the costs of our railways, so that both taxpayers and farepayers get a better deal.
"That is why Sir Roy McNulty is leading a review into reducing rail industry costs and why we have begun a wholesale review of rail franchising."
The DfT is currently consulting on whether strict obligations to avoid overcrowding, for example, by adding more carriages, should be imposed more widely on train operators.
Currently, Chiltern Trains is the only operator required under the terms of its franchise to bear the responsibility - and cost - of providing enough capacity to meet demand.