Chiltern Railways report major problems with no services running into Marylebone in London this morning because of a fire which has hit the entire signing system on the line.
Marylebone is closed. The problems could continue until tomorrow.
Services will start at Harrow or Ruislip with major delays and cancellations.
Heathrow Airport has revealed it is operating on a knife edge every day - because the site is so busy, that it sometimes works close to full capacity.
Managers have said they are now introducing radical new measures to reduce daily delays and the massive chaos caused during bad weather and when things go wrong.
The changes include the installation of computer technology used by Formula One racing. Our Transport Correspondent Mike Pearse reports.
David Cameron described travel disruption caused by the Underground strike in the capital as "unacceptable" on Twitter.
It's unacceptable that millions of people are having their lives disrupted by today's Tube strike in London.
Thousands of staff and volunteers are working hard to keep the capital working during the "pointless" Tube strike, London Underground said.
TfL said the Victoria line was now running fully with the Central line running to Holborn and the Jubilee line running through from Wembley Park to Stratford.
London Underground managing director Mike Brown said:
Thousands of staff and volunteers are working hard this morning to keep London working and our customers informed in the face of this pointless strike.
More London Underground staff have come to work this morning than during the strike back in February, and a record number of London buses are operating.
A Transport for London spokesman said the RMT could not "claim a shut down in any sense of the word."
Passengers formed long queues in London Bridge underground station on the first morning of the Tube strike.
Commuters have been told to expect severe disruption, with TfL advising passengers to plan ahead.
A 48-hour Tube strike has created "total chaos" for Londoners just trying to get to work, one commuter has said.
Emily Toner told ITV London that she was forced to hire a Boris bike because of the crowds at Liverpool Street station:
When arriving at Liverpool Street at the Tube, I discovered the gates were still not open so I resorted to using a Boris bike to get into work.
Total chaos has been caused to good people just trying to get to work.
Commuters have reported having to wait a long time for black cabs at Paddington station this morning.
Staff at the station warned passengers to expect lengthy queues yesterday.