In 2006, London Underground decided to replace the exisiting Metropolitan Line trains. Although the new trains involved a number of improvements, they also had fewer seats. That decision has meant that many commuters are no longer able to get a seat and, indirectly, has meant that 'fast' services are no longer available outside peak hours.
David has been campaigning on this issue for many years - highlighting the difficulties that the new trains would cause back in 2006. More recently, he has been campaigning for more trains at peak times, as well as more fast trains outside peak hours.