South Western Railway workers are holding a 24-hour strike in the ongoing dispute over the role of guards.
The company says it plans to deliver more than half of its normal Saturday service.
Members of the RMT Union say they want to "retain guards and protect safety, security and accessibility" on services.
SWR say the industrial action is "unnecessary and pointless" and their plans mean "more not fewer guards".
Union members have so far staged four out of eight planned 24-hour walkouts this summer.
Further strikes are planned on the following dates:
- Friday 31 August – Sunday 2 September (72-hour strike)
- Saturday 8 September (24-hour strike)
- Saturday 15 September (24-hour strike)
SWR is running additional services between Bournemouth and Weymouth, Portsmouth and Eastleigh and Alton and Woking.