Rail fares have increased twice as much as pay since 2010, new research suggests.
Commuters and other passengers will find out on Tuesday how much extra they will be charged from the new year.
The Government links the annual January rise in Britain's regulated fares with the previous July's Retail Price Index (RPI) measure of inflation, which will be announced by the Office for National Statistics (ONS).
Regulated fares make up almost half of all tickets and include season tickets and standard returns. They increased by 1.9% in January, but the RPI figure for July this year is expected to be around 3.9%, which would lead to the highest increase in fares since 2012.
An analysis by the Rail, Maritime and Transport union (RMT) showed that rail fares have risen by around 32% in eight years, while average weekly earnings have only grown by 16%.
RMT general secretary Mick Cash said: "The private operators and government say the rises are necessary to fund investment but the reality is that they are pocketing the profits while passengers are paying more for less, with rail engineering work being delayed or cancelled, skilled railway jobs being lost and staff cut on trains, stations and at ticket offices."
Fewer than half (47%) of passengers are satisfied with the value for money of train tickets, according to the latest survey by passenger watchdog Transport Focus.
TUC research showed that UK commuters spend up to six times as much of their salary on rail fares as other European passengers.
TUC General Secretary Frances O'Grady said: "This is grim news for commuters, who are facing another year of fare hikes. Overcrowded and understaffed trains are costing them more and more."
Shadow transport secretary Andy McDonald said the Government "simply can't justify allowing passengers to be ripped-off under privatised rail".
A Department for Transport spokesman said: "The Government carefully monitors how rail fares and average earnings change, and keeps under review the way fare levels are calculated.
"We are investing in the biggest rail modernisation programme for over a century to improve services for passengers - providing faster and better trains with more seats.
"Regulated rail fares are capped in line with inflation for next year."