Waterloo station was hit by rush hour mayhem on Tuesday night as dozens of trains were delayed.
South Western Railway said it was because a passenger fell ill on a train at nearby Vauxhall, blocking some London-bound trains.
Waterloo was particularly affected, with pictures from the scene showing hundreds of commuters trapped at the station. Waterloo is the busiest in the UK.
South Western Railway said: "Due to a passenger being taken ill at Vauxhall some lines towards London bound are blocked. Disruption is expected until 8pm."
Responding to a passenger on Twitter, it also said: "Hi, this incident happened only within the last hour however it blocked a platform for approx 15 minutes, the passenger was unable to be removed from the train for safety reasons. This has caused delays to services awaiting to pass through."
Customer information boards at the station also failed briefly this evening, adding to the confusion. South Western Railway confirmed it was not linked to the incident at Vauxhall.
This page is being updated.
