Commuters are facing major disruption at rush hour with trains between Clapham Junction and London Waterloo brought to a standstill.
South Western Railway said they have received an emergency call from a train between the stations and until the situation has been assessed all trains in the area will have to remain at a stand.
Some lines are blocked and staff are being sent to the scene in order to determine what action needs to be taken.
National Rail has warned disruption is expected until 1pm.
Meanwhile motorists are being warned of morning rush hour disruption following a serious crash which has closed the M26 in Kent eastbound between the M25 and the M20.
The motorway was shut following the accident involving a car and a van around 11pm on Tuesday night. A diversion is in place.
