Passengers frustrated by delays have jumped from trains and walked down tracks near one south London station.
Southeastern Rail said it was dealing with a "serious trespass incident" after the doors on a train were forced open while it was stopped outside Lewisham station on Friday evening.
Power has been turned off while staff ensure the tracks are clear prompting severe delays, the company said.
British Transport Police, Metropolitan Police and London Fire Brigade are in attendance.
A Southeastern spokesman said: "There are currently severe delays to trains in the Lewisham area following a serious trespass incident.
"Several passengers have forced open the doors on a train and disembarked while it was stopped outside of the station.
"We've had to turn the power off for safety reasons, and Southeastern staff are currently working closely with Network Rail and the Police to clear the trespassers from the tracks so that we can get trains moving again.
"We completely understand that passengers on delayed and busy trains may be frustrated, but they simply must stay on the train for their own safety.
"These trespassers risk being electrocuted by the 'third' rail or hit by other trains. They're also causing significant further delay for other passengers that our staff are working hard to get home."
British Transport Police said in a tweet that "all available officers" had been deployed to evacuate passengers trapped on trains in the area.
Mick Cash, leader of the Rail, Maritime and Transport union said: "RMT members are out there working in appalling conditions on our railways tonight and they deserve nothing but total respect and support.
"Tonight's incidents have once again raised some serious issues about the capacity of Britain's railways to cope with added pressures from adverse weather.
"There must be a full and transparent investigation into the serious issues that have been raised on on transport services over the past week. Warm words from Chris Grayling just won't cut it."