LONDON — It was part of the imagery of much earlier times: a red double-decker bus nudging through London streets shrouded in smog created in part by its own exhaust fumes.

Such was the vehicle’s lumbering notoriety that a musical duo popular in the 1950s and ’60s, Flanders and Swann, composed a tongue-in-cheek panegyric to the “London Transport diesel-engined 97-horsepower omnibus.” They called their song “A Transport of Delight.”

On Monday, though, the city’s 9,500 buses — still mostly painted red — laid claim to a fresher narrative.

While the worst smogs, or “pea soupers,” have long dispersed, London still chokes on heavy pollution. Seeking to curb toxic diesel fumes, transport officials and companies are hunting for new sources of energy for the buses.