The disruption at London Bridge Station is expected to last until the end of service tonight, National Rail have said.
Before tonight's rush-hour, Boris Johnson met executives from Network Rail. He said:
I know Network Rail are very apologetic for any inconvenience and delays passengers have experienced.
This is a vast project which will produce massive benefits for all travellers at and into London Bridge. It involves the total rebuilding of the oldest commuter railway station in the world.
There will inevitably be disruption from these huge improvements, but they have reassured us that they are straining every muscle to keep that disruption to an absolute minimum.
National Rail Enquiries says that the disruption is likely to last until at least 10pm.
Passengers are being allowed to use their tickets on buses between London Bridge and East Croydon, Purley and Caterham, on London Overground and London Underground services on 'reasonable routes' and the Tramlink between Beckenham Junction and East Croydon.
We apologise for disruption again tonight. If any minor consolation, we hope at least your experience on the station was improved tonight.
Commuters are facing a fourth night of disruption at London Bridge station.
Signalling problems are a causing delays to all Southern services using the station.
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Rail upgrades in the South East costing more than £2 billion will get underway this week.
Network Rail announced the investment to try and accommodate for increasing numbers of commuters coming into the capital from Kent and Sussex.
The 5 year plan to improve stations, introduce more trains and 20% more seats will get underway this week.
Some routes have seen a 50% increase in passengers over the past 10 years, so Network Rail now hopes to succeed in connecting Kent and Sussex with London before 2019.
This will form part of a wider programme to deliver an extra 115,000 seats a day into the capital.
Network Rail's Dave Ward, said: "The growth we are managing now is phenomenal - Southeastern and Southern already run more than 2,000 trains each every day - and this has given us a real opportunity to improve the railway in Kent, Sussex and south-east London."
Passengers using London Bridge have been warned to expect severe disruptions for up to four years as the station is redeveloped. Today Network Rail was showing off the first two new platforms to be completed.
They said the ongoing work would provide 'tremendous benefits' to passengers. But as Luke Hanrahan reports, for some it comes at a significant cost.