The Indian steam locomotive built in 1855 and listed by Guinness as the oldest in regular service was the Fairy Queen.
Also known as East Indian Railway locomotive No. 22, the Fairy Queen was constructed in Leeds, England, for the East Indian Railway Company. It reached Calcutta, now Kolkata, in 1855 and worked passenger, freight, troop, and railway-construction duties in India.
After retirement in 1909, the locomotive spent decades preserved as a historic object. It was restored to working order in 1997 and resumed heritage excursions between New Delhi and Alwar. Guinness listed it in 1998 as the world’s oldest steam locomotive in regular service.
The claim refers to its recognized operating status, not uninterrupted daily service since 1855. Another 1855-built locomotive, EIR No. 21 Express, has sometimes been discussed as an older contender, but Fairy Queen is the canonical answer to this Guinness-record question.