Train operators have been accused of topping up their profits by increasing the cost of car parking.
Hundreds of station car parks in England - many in the Home Counties - saw prices go up by at least twice the rate of inflation this month.
Commuters and motoring groups say the rises are excessive and bigger than increases in train fares.
Train firms say many stations have not seen price rises and others still cost around the same as nearby car parks.
More than 50 stations operated by the South Eastern rail franchise have increased charges from £3 to £3.50 this month, an increase of 16%.
East Midlands, First Capital Connect, First Great Western and South-West Trains have all put up charges by at least 8%, though not at all stations.
Such increases come on top of the increased price of fuel, and an average rise of 6.2% in train fares this month, adding to the misery for commuters.
Regulars catching a train from Three Bridges station in West Sussex got a particularly nasty surprise when they arrived at the station car park this week.
They found that charges had been increased by 19%, without warning, from £4.20 to £5.00.
"That's a shocking amount of money," said Darren Roberts, who does a daily commute to London.
"You're encouraged to be out of your car, but you get hit with money upon money upon money to get where you need to be."
As with many stations, commuters have nowhere else to park, and often cannot get a space anyway.
Southern Railways, which operates Three Bridges, says it is trying to encourage more people to use a second car park 100 metres further away.
It has raised charges there by much less than at its main car park.
Nevertheless prices at the second car park have still risen by 9%.
The RAC Foundation believes that train operators are putting up car parking charges as a sneaky way of increasing their profits.
While the price of train tickets is controlled by the regulator, rail companies are free to set their own charges for car parking.
"We suspect that they're topping up their profits," says Jo Abbotts of the RAC Foundation.
She points out that the price of parking is now as much as a quarter of the cost of the ticket itself.
A season ticket from Oxford to London costs £4,104, she says, while the cost of parking at Oxford station is £1,200 a year.
The RAC Foundation is particularly worried that the price of parking could encourage commuters to abandon the train, and drive to work instead.
"If the cost of car parking is preventing them from taking the train, and forcing them to take their cars for the entire journey, then we need to address those issues," says Jo Abbotts.
The train operators point out that many stations have not had price rises at all, or else they have adjusted charges to remain comparable with nearby car parks.
In Scotland there have been no increases, as the operator Scotrail is not allowed to put up prices without permission from the Scottish government.
Other operators say they have increased charges to pay for improvements, such as security lighting.
The train companies say they have also had to cope with the VAT rise this month, on top of CPI inflation, which is running at 3.7%
A spokesperson for the Association of Train Operating Companies said: "Many of the UK's stations don't charge for car parking at all, and many car parks have discounted prices at quieter times of the day."
But the RAC is still not convinced.
It suggests that car parking charges could be capped, and only allowed to increase at the same rate as train tickets themselves.
BBC News website readers have been sending in their reaction to this story. Here is a selection of their comments.
I've stopped using the train for my daily commute, preferring a 30 minute drive to my nearest underground station. Although parking charges have gone up there too, they are still much less than my local station, which is £6.80. The journey itself costs much less and I actually get to sit down! Nigel, St Albans, Hertfordshire
In December the daily charge to park at Gloucester station was £7.00, yesterday the charge was £8.20, a 17% increase. Such a charge, plus the cost of rail fare begins to make travelling by car an economic option. Steve, Gloucester, Gloucestershire
I believe the station car park in Fleet, Hampshire has had the biggest increases. The weekly car park fees have risen 25% from £24.00 to £30.00 per week. Jeremy Greenwood, from Fleet, Hampshire
Farnborough Main Station went up from £6 to £7. Graham Fradgley, Sandhurst, Berkshire
Its £8 per day at Birmingham International Train Station. Nic Mellor, Birmingham, West Midlands
Our local car park used to be £6 a day. It was in a dreadful condition, it was often closed and you had to park over a mile away and catch a shuttle bus. They built a new car park and that's now a whopping £8 a day! People complaining about a £3.50 a day charge should try living here. There is very little industry or jobs in this town, so the local train station have a monopoly on car parking. Local residents are now unable to park by their own houses because people are refusing to pay the £8 and blocking up residential side streets. It's all about making money for the operators which is all they seem to care about these days. Jess, from Stafford, Staffordshire
I live close to Ebbsfleet International which serves London and the Eurostar. Car parking there is an amazing £8 per day. Needless to say, the car parks are not that full. For a while, commuters were parking along a nearby road that didn't have parking restrictions and then walking to the station. Now it seems cars that park there get tickets but still no visible restrictions. High street, supermarket, railway, in fact any business should not charge for parking, after all, we have to get to where we need to and we will spend our money. Although public transport is not always available or the most efficient way to get to work. Kelly Sands, Dartford, Kent
In most other European countries, train and rail companies provide free parking for their customers as a matter of course. The fact that UK rail companies charge at all for parking, let alone the outrageous parking prices, is another manifestation of "rip-off Britain". I think it is time for commuters to refuse to pay and jam up the car parks with their cars! Jon Holland, Truro, Cornwall
I park at Ewell West which is just in Zone 6. The parking cost has gone up 10% from £5 to £5.50. Worse than that, I am an Oyster card user, which means I am not allowed to purchase monthly car park tickets or longer season tickets. If I was allowed I could save around 20% of my parking costs. They claim it is because I use Oyster - which is cheaper than a season ticket - and therefore could be using a space which one of their season ticket holders would use (which is nonsense). It's all about maximising their income, I travel cheaper by Oyster so they make me pay the maximum for the car park. Terry Haynes, Epsom, Surrey
I've been commuting to London for five years. East Grinstead station car park increase parking fees every year with no warning to passengers. Up until last year the increase was usually no more than 10p, then it went up by 50p to £4.70. This year, it went up a further 40p to £5.10. To add insult to injury, and despite several complaints this year and last, they made absolutely no effort what so ever to clear the area of ice and snow, the ground was treacherous, and people had great difficulty getting their cars out of their spaces at the end of the day with many having to dig them out or call for help. There are 236 spaces in this particular car park. Lorraine Ilott, East Grinstead, West Sussex
Car parking charges for a day at Reading Station have risen from £18 to £20, well above the rate of inflation. There have been no upgrades to this car park and prices at surrounding car parks, such as Queens Road is approximately £12 for a day. None of the justifications provided by the train companies appear to fit with these facts. I simply do not understand how this can be acceptable when the general idea is to keep people out of their cars and on public transport. A standard return ticket to London costs £38, plus car parking at £20 means £58 per day. I can fill my car's petrol tank to the brim for that money even with today's pricing. Factoring in a season ticket reduces the cost to (being generous) £29 per day. That's still a full tank of fuel every two days - its ridiculous. Dan Creswell, Reading, Berkshire
It's not just train stations though. I have seen commuters voting with their feet and switching to the Underground. High Barnet car park fees have gone from £3.20 last year to £4.50 this year, a 71% rise but what can I do? I have to get into work somehow. Matthew Martin, London