LONDON—Tourists waiting for their morning subway train to Madame Tussauds were treated to an unusual sight Sunday: a 19th- century steam engine chugging down the tracks.

Transit officials sent the Met Locomotive 1, built in 1898, down London's Metropolitan Line to mark the 150th anniversary of the capital's Tube network, the world's oldest.

Hundreds of train fans, costume-wearing enthusiasts, and curious onlookers gathered at platforms and bridges across the city to watch as the locomotive traveled non-stop from Kensington Olympia station in the west to Moorgate station in central London.

London Mayor Boris Johnson was among the invited passengers aboard the historic black-and-red locomotive. He said the trip was "romantic," describing "thick clouds of white steam going past and then bits of soot coming through from the engine."