Brighton Station closed over overcrowding fears
Brighton station was closed amid fears of dangerous overcrowding and claims commuters had fainted in the hot weather.
Signalling failures around Gatwick Airport, as well as a hole in the south-east London area caused delays and cancellations through the day.
Brighton Station was closed temporarily at 19:20 BST to allow crowd control measures to be put in place.
Southern said it had not received reports of passengers being taken ill.
Crowds formed at Brighton station on Tuesday evening, at the end of 2016's hottest day so far.
Passengers reported being stuck on trains in sweltering heat, while others said they were crammed onto station concourses.
Water bottles were being handed out at the station as some passengers claimed air conditioning on board some services between Brighton and the capital was not working.
Jo Edwards tweeted: "At Vict - no trains to Brighton, no advice/updates, poor lad has just fainted & being fanned by his dad, third world service".
Others were critical of operator Southern, which has recently been hit by widely-publicised services troubles and an ongoing industrial dispute.
Toby Robertson wrote: "Get rid of these clowns. It's dangerous. Short form train, people fainting."
MP for Hove and Portslade Peter Kyle has said he will make the anger felt by those suffering a terrible evening at the station known to the government.
And Brighton MP Caroline Lucas tweeted: "So sorry & angry about chaos at #Brighton station & on trains - have raised serious safety concerns & seeking urgent meeting with Minister".
Southern, which said it ran three services between 20:20 and 20:40 to help ease congestion, said Network Rail were working to fix the points failure in the Gatwick area.
A spokeswoman said: "We've had no reports of any passengers being taken ill or fainting however social media would suggest otherwise and we will investigate this further.
"We do of course apologise to any customers that have experienced any issues with overheating on our services today."
It comes as commuters have experienced months of cancellations and delays on Southern trains due to an RMT dispute.
The train operator has axed 341 trains of its 2,242 weekday trains due to staff shortages.
New rail minister Paul Maynard will appear before the Commons Transport Select Committee on Wednesday to give evidence on the Department for Transport's role in the issue.