Earlier this week, a plan by SWT to scrap some tickets offices was blocked
A leaked memo has revealed a train company is running nearly 100 shorter trains a day to save money.
South West Trains (SWT), which runs about 1,700 services daily, said the decision would not hit rush-hour trains but would affect daytimes and weekends.
A SWT spokeswoman said shorter trains used less electricity so it was "good housekeeping".
A watchdog broadly backed the plan but said passengers must not be "inconvenienced" by any overcrowding.
Jocelyn Pearson, of Passenger Focus, said: "If this saves money without inconveniencing passengers then we have no objections.
"We would want to see what times these trains are running and passenger figures on these routes.
"I believe the plan is to uncouple trains, whereas at the moment the full carriage lengths run on each route."
SWT operates routes through Hampshire, Surrey, Dorset, Wiltshire, Berkshire, Devon, Somerset, Cornwall and London.
Rolling stock campaigner David Habershon said: "These daytime and weekend services are not empty, not at all. It is nothing more than a cost-saving measure without consulting the passengers."
The memo, leaked to BBC South, read: "Most changes relate to our need to control costs, which has resulted in a number of train formation changes.
"Some of those concerned are already in place on a STP (short term plan) basis, but will now become permanent."
A SWT spokeswoman said the programme had already started and would roll out fully in the next few weeks.
She added: "We are looking at trains at weekends and daytimes where there are fewer passengers.
"During school holidays we will reinstate the carriages and we already have extra services operating during special events. It is purely good housekeeping."
Earlier this week, another cost-cutting plan by SWT to reduce opening times at ticket offices was mostly rejected by the government.
Transport Minister Lord Adonis called some of the proposals "unacceptable" and said tickets should remain easily available.