The Ladbroke Grove rail crash (also known as the Paddington rail crash) was a rail accident which occurred on 5 October 1999 at Ladbroke Grove in London, England, when two passenger trains collided almost head-on after one of them had passed a signal at danger. With 31 people killed and 417 injured, it remains one of the worst rail accidents in 20th-century British history.It was the second major accident on the Great Western Main Line in just over two years, the first being the Southall rail crash of September 1997, a few miles west of this accident. Both crashes would have been prevented by an operational automatic train protection (ATP) system, wider fitting of which had been rejected on cost grounds. This severely damaged public confidence in the management and regulation of safety of Britain's privatised railway system.

A public inquiry into the crash by Lord Cullen was held in 2000. Since both the Paddington and Southall crashes had reopened public debate on ATP, a separate joint inquiry considering the issue in the light of both crashes was also held in 2000; it confirmed the rejection of ATP and the mandatory adoption of a cheaper and less effective system, but noted a mismatch between public opinion and cost-benefit analysis.

The Cullen inquiry was carried out in two blocks of sittings, sandwiching the 'joint inquiry'; the first block dealt with the accident itself, the second block dealt with the management and regulation of UK railway safety; this had always been part of the inquiry terms of reference, but was given additional urgency by a further train crash at Hatfield in October 2000. Major changes in the formal responsibilities for management and regulation of safety of UK rail transport ensued.