Guards on South Western Railway (SWR) are staging a 24-hour strike in the long-running dispute over their role.
The company said six in 10 trains would run during the industrial action by the RMT union.
A revised timetable is in operation and passengers are advised to check before they travel.
It is the first of eight strike days planned throughout the summer, with the RMT saying its members were "rock solid and united" behind the action.
Replacement buses are operating on routes, including between Salisbury and Exeter, through Camberley and between Aldershot and Guildford.
Passengers vented their frustration on social media.
Train strikes in this weather just make trains busier and dangerously hot. We pay thousands of pounds for this service - please stop making up suffer 😩#strikes #SWRStrike #earlsfield #train #Heatwaveuk— Tiffany Outred (@TiffanyOutred94) July 26, 2018
Deafening silence from @SW_Help today in regards to #SWRStrike. No information at stations, with constantly changing timetables from @nationalrailenq Shocking service with no thought for your paying customers who have had to put up with a shoddy service since you took over!!— notstillinmy40s (@tracyg_66) July 26, 2018
@SW_Railway Another day, another strike. The least you could do is warn us but there were absolutely no signage or announcements. First I heard was from BBC news today. 5 stars from SWR once again 👏🏼 #SWRStrike— Alex (@AJ_Sheppy) July 26, 2018
RMT general secretary Mick Cash said the action was about protecting "safety, security and accessibility".
He said: "RMT is angry and frustrated that a request by the union to get serious talks with South Western Railway bolted down this week was met with the same old tired response from the company that refuses to move the process on and address the fundamental issues."
Workers are due to strike for eight days over six weeks in the long-running row over the role of guards on trains.
The RMT has said changing the role of guards and extending driver-only services would compromise staff and passenger safety, which the government denies.
Analysis - Paul Clifton, BBC South
It's the first strike on SWR since 12 January. The RMT has called two strikes since then, but cancelled them when talks showed signs of progress.
No such luck this time: today is the first of eight strikes across the summer.
South Western says almost two in three trains are running today. That still means around 600 services are being cancelled.
The train operator is prioritising the big passenger flows on the main lines: Southampton, Portsmouth and Reading to London Waterloo and is adding extra carriages where it can.
It's the peripheral routes that lose out: Salisbury to Exeter and Bristol, the line through Camberley and from Aldershot to Guildford.
It helps that school holidays have started as commuter numbers always dip in late July.
SWR said the walkout was "frustrating and disappointing".
Managing director Andy Mellors said: "The issue we're talking about here is how we deal with the exceptional circumstances when a guard may not be available because of short notice disruption and wanting to do the best thing for out customers and keep people moving."
Further strikes are scheduled to take place on 28 and 31 July as well as on 4, 11, 18, 31 August and 1 September.