Transport for London are offering rare walking tours through the London Overground Thames Tunnel this weekend, while trains are suspended for engineering work.
The £18 ticket cost will go to charity - but all 4,000 sold out in just 4 hours.
The tunnel was constructed in the 18th century, and it's only the second time in 150 years it has been open to the public.
There is no service between Gospel Oak and Barking while attempts are made to fix a faulty train at Gospel Oak.
TfL say tickets will be valid on the buses and the Underground.
The Rail Maritime and Transport workers union has confirmed that a strike on London Overground services will go ahead this weekend after an agreement was not reached over job cuts.
The RMT claims that 130 guard jobs are at risk over plans to run driver-only services on some trains. They will walk out for 48 hours from 00:01 on Sunday August 25th.
The strike takes place over the weekend of Notting Hill Carnival and is expected to cause widespread disruptions to services, especially on the Richmond and Clapham Junction to Stratford Junction routes.
Transport for London said they were working hard to keep as many services running as possible and there will be alternative Tube, bus and rail services available.
London Overground has released a statement, following news that the RMT union is planning a series of strikes in protest at the removal of guards from its trains.
Rail union RMT has announced that guards on London Overground will strike on the following days:
Sunday 25th August
Monday 26th August
The strike is over a move by the company to introduce Driver Only trains (without guards) on the whole of the London Overground network.
The rail union RMT is threatening strikes on London Overground in a row over plans to axe more than 100 guards. Transport for London wants to switch to driver-only trains on the North London Line from Stratford to Richmond and Clapham.
TfL says 130 posts will disappear, saving £5m a year. RMT General Secretary Bob Crow said: "The news that millions of passengers are to be put at risk through plans to throw guards of London Overground trains on north London routes will send shockwaves through transport services."
Transport for London insisted the guards are no longer needed to open and close doors and the move would bring services in line with other parts of the Overground network.
Acting Director of TfL London Rail, Jonathan Fox, said: "The safety of our customers and staff is paramount and these proposals will maintain our excellent safety record."
The are severe delays on the London Overground between Stratford and Richmond due to a road vehicle hitting abridge. Tickets are being accepted on London Underground, London Buses andDocklands Light Railway services. Source: Inrix
More than 500 workers on the London Overground have been awarded a four-year, above inflation pay deal.
Staff will receive a 3.85 percent increase this year followed by three further rises.
The RMT union says the deal sets a benchmark for the rest of the industry.