Network Rail and Thameslink bosses face the London Assembly's Transport Committee later.
Rail executives will be questioned over recent overcrowding and chaos at London Bridge station.
Commuters faced weeks of disruption because of reconstruction work at the station.
London Bridge Station has again been struck by rush hour problems, on the day new measures designed to ease commuter congestion were due to come into force.
Train services were affected by signal failures between 4.30pm and 5.30pm but the knock-on effect was due to affect trains until around 7pm, , Network Rail said.
It comes as the station undergoes extensive long-running engineering work which has led to major congestion, including one occasion when police were called to keep order.
Network Rail tweeted: "London Bridge station is extremely busy - please consider using alternatives this evening."
A spokeswoman added that they were advising people to use Victoria instead, saying: "A signal failure outside the station put two of the four lines in to London Bridge out of action between 4.30pm and 5.30pm. We fixed the problem but as with anything like this there is a knock on effect."
The Rail Minister said it was not a matter of crowd control but passengers "trying to get home after a long day at work".Read the full story ›
Train services are expected to be disrupted until 9pm tonight, following delays between London Euston / Clapham Junction and Milton Keynes this morning.
Safety checks being made on the line between Hemel Hempstead and Watford Junction mean trains are having to run at a reduced speed between Milton Keynes Central and London Euston / Clapham Junction.
London Midland and Southern railways are no longer affected by this, but Virgin Trains will run a reduced service between Manchester Piccadilly / Birmingham New Street and London Euston.
Passengers may be delayed by up to 20 minutes.
The rail operator has said it let passengers down, after thousands experienced delays over Christmas as engineering works overran.Read the full story ›
Commuters in and out of London's Euston station are facing delays of up to 45 minutes until midday as a result of safety checks on the line and a signalling problem in the Milton Keynes area.
According to the National Rail Enquiries website, checks are being made on the line between Hemel Hempstead and Watford Junction which means trains running at slower speeds between Milton Keynes Central and London Euston / Clapham Junction.
There is also a signalling problem near Milton Keynes Central.
Until noon, Virgin Trains will run a reduced service between Manchester Piccadilly / Birmingham New Street and London Euston.
No Southern trains will run between Milton Keynes Central and Clapham Junction and a reduced service will run between Shepherds Bush and Clapham Junction.
London Midland have halted London Euston to Tring services. Alternative services will make additional stops at Apsley and Kings Langley.
Commuters in South East London are bracing themselves for travel disruption as major changes to Southeastern services begin today.
Southeastern's Charing Cross trains are no longer calling at London Bridge until August 2016 as the next stage of the station's £6.5bn Government-sponsored Thameslink Programme gets underway.
Passengers who need London Bridge will have to catch a Cannon Street service or interchange enroute.
Charing Cross services to and from New Cross, St Johns, Deptford, Greenwich, Maze Hill and Westcombe Park will be permanently diverted to and from Cannon Street.
There will also be fewer peak trains - two fewer trains in the morning and four fewer trains in the evening peak.
The line will be served by longer trains to boost capacity in peak hours.
Transport for London has warned that further modernisation works are to start at London Bridge on Monday which will cause more disruption for passengers trying to use the station. There have already been severe problems there this week after works meant the train timetable had to be changed. From Monday, there will be several new changes to train services:
- Southeastern rail services to and from Charing Cross will not call at London Bridge.
- Southeastern services from New Cross, St. Johns, Deptford, Greenwich, Maze Hill and Westcombe Park stations will no longer operate to and from London Charing Cross or Waterloo East, and will instead operate to and from London Cannon Street.
- No Bedford to Brighton Thameslink trains will call at London Bridge station until January 2018.
Passengers are being warned to avoid Canada Water station at peak times if they can and Underground and Overground stations in the area will be much busier. Extra buses are being added to routes 21, 47 and 381 and Cannon Street station will be open for longer.
We are accepting tickets at key stations from passengers whose journeys to London Bridge are affected, and deploying extra staff to assist customers and putting on dozens of extra bus services out of London Bridge and Waterloo. We will continue to keep all our customers informed with regular travel information via our staff, website and social media. As always, if you have the flexibility to do so, please think about timing your journey outside the busiest times.
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.