We are recreating a car and train crash in Bromley as part of a major training exercise on Sunday, 21 June.
The exact details of the exercise are being kept secret, until it starts on Sunday at 1200, in order to ensure the day is as realistic as possible for everyone taking part.
Around 250 emergency service personnel, staff from South Eastern Trains, Network Rail and other volunteers will be taking part in the exercise, which is expected to last until 1700.
Richard Welch Borough Commander for Bexley said: "I wanted to warn residents in advance as the scenario we are testing will look pretty realistic.
"There are going to be a lot of firefighters, police officers and ambulance crews in the area but we will endeavour to keep disruption to a minimum.
"Whilst this scenario may appear worrying and rail crashes are extremely rare, exercises like this are a vital way of practising our skills and ensuring that we are ready should there ever be a major incident in the capital.
"Days like this are a chance for firefighters to practise search and rescue skills as well as testing how we work with other agencies in the event of a major incident."
The exercise will have two locations: a road traffic accident on Lansdowne Road Bridge, just North of Sundridge Park railway station, and a train derailment in-between Sundridge Park railway station and Bromely North station.
The train crash will be on a section of line not used on a Sunday, so train services will not be affected. Lansdowne Bridge will be closed to pedestrians on the day.
Volunteers from the Railway Industry First Aid Association
(RIFAA) will play casualties in need of rescue on the day.
The scenario was devised by London Fire Brigade with support from Network Rail, South Eastern Trains, Southern Rail and the British Transport police.
London Ambulance Service, The Metropolitan Police Service and London Borough of Bromley are also taking part in the training.