Only days after train fares rose by an inflation-busting average of 4.2%, rail firms have been accused of "ripping off" passengers by increasing car parking charges at stations
An annual car parking ticket at High Wycombe, Buckinghamshire, has increased from £960 to £1,040, an 8% rise, while a weekly ticket to park at Hitchin, Hertfordshire, has gone up from £31 to £32, said the union.
TSSA general secretary Manuel Cortes said passengers were being treated as "cash cows" by train operators.
The rises come shortly after much criticised increases in rail fares
"These firms are squeezing passengers until the pips squeak. While fares are controlled by ministers, there is a free-for-all on car parking charges. Rail firms simply charge what they think they can get away with.
"It is a scandal that they are increasing charges by as much as five times the rate of inflation. This is daylight robbery at a time of falling living standards."
The union has called for car parking charges to be regulated in line with fares as part of the rail ticket price review currently being conducted by ministers.
The union's complaint is the latest row to hit the rail industry in the past week.
Commuters, unions and campaign groups joined forces last week to protest at the increase in rail fares, while rail minister Simon Burns is under fire after it emerged he shuns commuting by train in favour of a chauffeur-driven government car.
The Conservative MP was criticised for travelling the 35 miles between his Essex home and Westminster using an £80,000-a-year departmental car service.
Weekday parking fees at London Underground stations have increased by an average of 18.73% - going up from £4.27 to £5.07.
Transport for London said the increase was the first for more than two years and would bring prices more into line with other local car parks.
A spokesman for the Association of Train Operating Companies said: "Many stations don't charge for car parking and where operators do put up prices, much of the money raised goes towards the running cost of the railways in line with the government's policy to increase the contribution from passengers and reduce the share from taxpayers."
AA president Edmund King said: "Excessive parking charges at stations will drive some train commuters off the rails. Many car/rail commuters have no alternative and are now being shot by both sides with hikes in rail fares and parking charges.
"Drivers will not be encouraged to get out from behind the wheel if they can't afford to park their wheels."