A fire that broke out on the upper level of the Harvard Square subway station caused evacuation of an inbound train and 20-minute delays to and from the station during rush hour yesterday morning.

The fire--which shot sparks as high as the station's ceiling--began at 9:22 a.m. at the east end of the station when a short circuit ignited the "live" 600-volt rail on the subway track.

Massachusetts Bay Transportation Authority (MBTA) officials noticed the fire only after the inbound train had pulled to within 10 feet of the flames, an MBTA engineer who asked not to be identified said yesterday. He added that officials backed the train up and then evacuated the passengers, none of whom were injured.

MBTA officials immediately shut off the electrical currents and suspended service to and from Park Street for 20 minutes, while Cambridge Fire Department officials tried to extinguish the flames. "If they didn't cut it off right away, it could have caused an explosion," the MBTA engineer said.

MBTA ran additional buses between the Park Street and Brattle Station stops and two express shuttle trains until 11:05 a.m., when the fire was extinguished, Paul Dinatelli, an MBTA spokesman, said yesterday.

A crowd of about 500 persons was standing next to the tracks when the fire occurred, Carl Shapiro, proprietor of the news stand in the station, said yesterday, adding that sparks also fell to the lower outbound level of the station.

Although Shapiro said a similar fire occurred about a month ago, Dinarelli said such fires are "uncommon."

The MBTA engineer said he believed yesterday's fire was bigger and more dangerous than last month's fire.

Shapiro said he was not alarmed by the fire. "Once you've been down here for a while you're used to everything," he said.