A person has been hit by a train near Brookwood, which is causing delays of up to an hour for trains between Basingstoke and Woking.
There is no firm estimate yet of how long disruption will last but it is likely to continue until at least 18:00.
Services are unable to call at Hook, Winchfield, Fleet or Farnborough.
A replacement bus service has been requested
Work has begun on a £7m project to improve the busy Wimbledon junction -to improve the journeys for thousands of passengers.
Journeys through the station for the next 6 weekends will be affected so that South West Trains can enhance rail reliability.
Work at the station begun at Christmas and will continue every weekend until mid-February, with no trains calling at Wimbledon on weekends.
We have scheduled the work in order to minimise disruption to passengers as much as possible but if you, friends or family are travelling during these times, I strongly suggest checking travel details first.
“This work will have a significant impact on our services and we apologise for the inconvenience caused but they are a vital part of improving our network and our service to passengers.”
Plans to transform rail services on South West Trains have been unveiled.Read the full story ›
Plans to tackle the problem of overcrowding on South West Trains have been revealed today and South West Trains said if it they don't go ahead "customers will not be able to get on trains anymore".
Transport correspondent Mike Pearse spoke to Jake Kelly from South West Trains.
"If we don't get it, ultimately customers will not be able to get on trains anymore, it's that important," Mr kelly said.
New platforms at Southampton Central and Guildford along with new flyovers to take trains over congested junctions at Basingstoke and Woking head a long list of plans being put to the Government.
Signalling problems at Farnborough Main are causing delays of up to 50 minutes between Woking and Basingstoke.
Disruption is expected to continue until 15:00.
Passengers diverted from southampton are crammed on a platform at Basingstoke waiting for a delayed train to Waterloo.
South West Trains said cancellations and short notice alterations should be expected for some time.
Train services running through Southampton station may be cancelled, delayed by up to 60 mins or revised at short notice. Disruption is expected until 4:00pm.
Signalling problems at Farnborough Main are causing delays of up to 35 minutes between Woking and Basingstoke.
Replacement Transport has been arranged and will operate between Fareham and Southampton Central and Eastleigh and Southampton Central.
Mutual ticket acceptance has been agreed between South West Trains, Southern, CrossCountry & First Great Western in the area. Customers may use any operators services to complete their journey.
Southampton passengers who were told to take the diesel CrossCountry service are now stranded at Basingstoke due to signalling problems.
Train services running through Basingstoke can expect delays at short notice. There is currently no estimate for when services will resume as normal.
There are delays to South West Train services at Farnborough Main due to signalling problems. Basingstoke bound lines at Farnborough are currently blocked.
Train services running through this station may be delayed at short notice. There is currently no estimate for when services will resume as normal.
Engineers are investigating the problems.
Mutual ticket acceptance has been agreed between South West Trains, Southern, CrossCountry & First Great Western due to disruptions and delays on train services running through Southampton.
Customers may use any operators services to complete their journey.