Four wheels of an air-conditioned Metro rake apparently started turning opposite to the motion of the train on Tuesday morning, resulting in friction that led to smoke, a burning smell and the evacuation of panic-stricken passengers.

Metro sources said the smell alerted the driver while the Dum Dum-bound train was entering Tollygunge station and he immediately called the control room.

Underground services were stalled for 30 minutes from 10am, leading to the cancellation of five pairs of trains in the morning rush hour. Services resumed after the crippled rake was removed.

Traction motors, which move the wheels, are kept operational in six of the eight coaches in a Metro rake. Two coaches — called trailer in railway terms — are pulled by the six powered coaches. According to a Metro source, the snag-hit coach, fifth from the front, was “non-powered”.

“Earlier, there was a problem with this coach and so the power supply to the traction motors had been suspended and it was being used as a trailer coach,” said the source.

There are four pairs of wheels in a coach and each pair is moved by a traction motor. “Two of the traction motors in the non-powered coach became operational all of a sudden on Tuesday, and the four wheels started moving in the reverse direction,” said the source. The friction the reverse motion caused led to emission of smoke from the train’s underbelly.

Sources said engineers from the Integral Coach Factory, Perambur, where the two AC rakes were built, and Metro officials were trying to find out what caused the reverse motion.

The new Metro general manager, P.B. Murty, said: “I don’t know of wheels running in the reverse direction. We are trying to find out what exactly went wrong. Till the experts give the train a fit certificate, it will not be run.”

The two AC rakes have thrown up a series of problems since arriving two months ago. Most of the problems were related to their braking system and sluggish doors during overcrowding.

“The trains are being run commercially without proper trial runs and troubleshooting. They should be immediately withdrawn for safety reasons,” said Dilip Mukherjee, general secretary of the Metro Railwaymen’s Union.

Metro is now left with 18 operational rakes. The one that got derailed on October 20 is yet to start running.