Campaign groups have called for more pressure to be put on the government to release funding for improvements to railway services in the west country.
Chris Irwin from Travel Watch South West says rail passengers are being short-changed by a service which often falls below an acceptable standard.
Transport 2000 claims First Great Western (FGW) has cut its number of carriages from 133 to 104.
The train operator says it is reviewing its services.
They say there are many historical problems with the infrastructure to sort out and some of its rolling stock is currently out of service because of maintenance work.
The train operator launched a major shake-up of their timetable in December after the company won the Greater Western Franchise and had to combine its services with those previously run by Wessex Trains.
Andrew Griffiths from First Great Western said: "What we have committed to do as part of the franchise obligation is to provide the same number of seats in the evening and the morning peaks by using the high speed trains more effectively."
Labour MP for Reading West Martin Salter said: "I think First Great Western should be given a very short timescale to put this situation right in order to solve problems that are entirely of their own making.
"If they don't, I think the franchise should be awarded to someone else."
But Mr Irwin said: "Unless the MPs hit the phones this weekend, then my guess is there's going to more cuts to come."
The government told BBC News they want the railway network to grow to keep up with demand.
A long term strategy is not due until next year.