A number of rail projects are set to be in the firing line in the government's Spending Review.
The BBC understands a £1bn plan to electrify the Great Western Main Line between London and Swansea, through Berkshire and Oxfordshire, may be axed.
A contract for a new fleet of trains to replace the 35-year-old carriages through Reading could also go.
One passenger group said the area would be hit hard. The Government said nothing had been decided.
Plans to electrify the Great Western Main Line - which would also benefit Wiltshire and Bristol - were attacked by the Conservatives when they were announced by the previous Labour government last year.
While plans to replace Britain's aging fleet of intercity trains, which run through the Thames Valley, were put on hold earlier this year and could be axed altogether in the review.
Chris Irwin, chair of TravelWatch SouthWest, said: "I think we should be worried, we are looking at something like a 40% cut in the money available to subside bus services and train services.
"There are question marks on electrification and replacing the 35-year-old trains, but that is just the start of it.
"We are going to see in the next few years fares up.
"We are going to see less reliable services if the 35-year-old trains aren't replaced. The Thames Valley will be hit hard."
BBC South's transport correspondent Paul Clifton said: "We were promised last year a new generation of intercity trains.
"If that is cancelled, and almost everyone thinks it will be, are we going to have to survive with these trains for another 20 years?"
He also said many in the industry thought the electrification was also at risk in light of the tight economic situation.
A Department for Transport spokesman said: "Decisions on the future of the Intercity Express Programme and the electrification of the rail network will be made in light of the Spending Review announcement [to be made on Wednesday]."