New services, extra seats, car parking and ticket machines have been announced as part of a £50m investment in South West Trains (SWT).
The funding follows an agreement between the franchise holders Stagecoach Group and the Department for Transport.
The company described it as the “biggest package of customer-focused benefits for years”.
Rail unions dismissed the announcement a “pre-election publicity stunt”.
SWT, which operates trains out of London Waterloo, said there would be 73 new evening and weekend services, more than 57,000 additional seats, 1,400 more car parking spaces at stations and 170 new ticket machines.
The new services will include a new early morning train from Southampton to London on weekdays, as well as a half-hourly service between Salisbury and London and on the Hounslow loop, on Sunday afternoons.
SWT said it would also have more staff on station concourses and planned to expand its smart ticketing programme.
Rail Minister Claire Perry said the funding would make a “huge difference” to commuters.
“As we spend record amounts on modernising our railways as part of our long-term economic plan, our franchising programme is also unlocking considerable investment from train operators,” she added.
Mick Cash, leader of the Rail, Maritime and Transport union, said: “This is yet another pre-election stunt on a basketcase franchise which has a shocking reputation for reliability and overcrowding.
“This £50m sounds like yet another sweetener for the private train operator and will have them laughing all the way to the bank.”
The new services are due to come into effect at the end of the year, with other passenger improvements introduced by February 2017.