DARLINGTON, England — The A1 Steam Trust is investigating the cause of a mechanical failure of its 10-year-old 4-6-2 Tornado on the locomotive’s first 90 mph passenger excursion, and the date of its return to service is unknown.

The locomotive suffered a breakdown about 43 miles north of London on April 14 on a run from London to York. The failure occurred as the locomotive and its train were heading north on the East Coast Main Line at about 90 mph.

According to reports, the locomotive stopped near the town of Sandy, and video at the scene showed the union link, a part of the valve gear, lying on an adjacent track. The initial inspection shows that the center piston valve on the three-cylinder locomotive overheated and seized, causing the combination lever of the Walschaerts valve gear to break. Trust officials have not pinpointed the cause, but they believe lubrication failure was to blame. No one was injured in the incident, although several following trains were delayed while a diesel was summoned to tow the locomotive and its train to Peterborough.

The engine, the first new steam locomotive built in Britain since the 1960s, had undergone winter maintenance and has operated a number of miles since without trouble, according to a statement by the trust. Speed did not appear to be a factor in the failure.

Passengers on the train were given vouchers for a future trip, perhaps over the same route, and they will be given first chance to book tickets. Because of the breakdown, several scheduled tours and appearances at railway museums have been postponed until the locomotive is repaired.