Online claims that a subway train was damaged in an explosion on Wednesday morning were quickly denied by Beijing Subway Company, which said the "blast" was nothing more than the noise of someone dropping a metal ruler onto the "third rail".

"It (the noise) was caused when a metal ruler dropped by a passenger caused the train's current-collector to short-circuit, which made the sound," the subway company explained in a statement posted on its website.

Passengers on Line 1 said they heard a "big bang" shortly after the train arrived at the Military Museum station.

Posts in online chat rooms and forums said a small explosion happened at around 8:40 am. Passengers said they were soon evacuated from the train that had been heading from Pingguoyuan station toward Sihui.

Some posts on sina.com's microblog said there had been an electrical discharge and even claimed that the last carriage had caught fire and that staff at the station used fire extinguishers to put out the blaze.

But the statement issued by the subway company said the incident was much less dramatic. It said the current collector - which is commonly referred to as the "third rail" can make a sound and even produce smoke if it comes into contact with a metal object that causes a short-circuit.

The company said the incident happened at 8:44 am and the train resumed normal operations four minutes later.

"Because the peak hour had already passed, there was not a major impact on the operation of the subway," the statement said.

The company reminded passengers to take care of possessions - including umbrellas, knitting needles and other long items that contain metal - while riding the subway.