Around 650,000 passengers are facing disruption on South Western Railway as the first of eight strikes by the RMT union begins.
The long-running dispute centres on changes to the role of guards on trains which the union say is unsafe, which SWR strongly deny.
Members have walked out for 24 hours, with the next strike on Saturday.
Six hundred trains have been cancelled, affecting all routes.
On a normal day the company runs 1,700 trains.
The company say most routes will have a reduced service with buses replacing trains on some lines.
Andy Mellors, Managing Director, South Western Railway
RMT strike action is planned on the following dates:
0001 Hours and 2359 Hours on Thursday 26 July 2018
0001 Hours and 2359 Hours on Saturday 28 July 2018
0001 Hours and 2359 Hours on Tuesday 31 July 2018
0001 Hours and 2359 Hours on Saturday 4 August 2018
0001 Hours and 2359 Hours on Saturday 11 August 2018
0001 Hours and 2359 Hours on Saturday 18 August 2018
0001 Hours on Friday 31 August 2018 and 2359 Hours on Saturday 1 September 2018
All the latest updates can be found on the South Western Railway website.
We are doing everything we can to minimise disruption caused by the RMT strikes and plan to run at least 60% of services on Thursday 26 July. We are publishing detailed service information, including on alternatives for passengers, on our website and will be updating this for future strike dates as soon as details are confirmed. We advise all customers to check their journey before they travel."
”RMT members are standing rock solid and united again this morning across South Western Railway in the latest phase of the campaign to put public safety, security and access before private profit.