Rail commuters from Brighton, Hove and Portslade were being warned that it may take longer getting in and out of London stations today (Tuesday 10 July).
And during the Olympics they should avoid using Victoria and London Bridge, Network Rail said.
Staff at five stations – Victoria, London Bridge, Cannon Street, Charing Cross and Waterloo East – are rehearsing procedures for managing crowds during the Olympics.
Diversions and cordons to direct queues will be tested and some passengers may have to come in and leave stations by different routes.
London Bridge is regarded as the station that is most likely to be a cause for concern.
One of the aims at London Bridge will be to manage spectators at equestrian events as they return from Greenwich Park and city workers as they head home at the same time.
Passengers getting off incoming trains will be diverted around the station to reach the entrance to the Underground.
Network Rail said: “During the Games some of our stations will be extremely busy at certain times of day – nowhere more so than London Bridge.
“We have plans to help manage crowds and keep people moving.”
The tests today were intended to give commuters the chance to get used to the way some of the busiest stations would work during the Games, Network Rail added.
And it would allow staff and managers to check that the plans were right.
The company added: “While commuters should continue to use London Bridge station as normal in the run up to the Games, our advice is to avoid the station if possible at Games time.”
