Better door maintenance by Alstom, the French company that looks after its trains, is one of several factors that have led to a turnround for the Northern Line, a long, complex line that carries more passengers than any other in London. Passengers dubbed the route, which includes the world's oldest section of deep-level tube tunnel, opened in 1890, the “misery line” in reference to its delays and overcrowding.

Such perceptions may be hard to dislodge. Indeed, managers at all three organisations involved in operating the line – Alstom; Tube Lines, the private consortium that maintains the line's infrastructure; and public-sector London Underground, which employs drivers and signallers – accept there remains considerable room for improvement.

However, the line has become an example of how public-private partnerships and private finance agreements can produce benefits when the participants work together. It also illustrates the powerful effects of an overhaul of financial incentives and re-examination of long-established working practices on the apparently intractable problems of a complex transport system.