An estimated 10,000 people fled surburban homes today after a locomotive sideswiped a chemical tank car in a rail yard three miles from downtown Boston, releasing clouds of acid that sent a least 120 people to the hospital.

The clouds of hydrocloric and phosphoric acid forced the closing of a section of Interstate 93 and of area businesses and schools.

About 3,000 people were evacuated from their homes in Sommerville and adjacent Cambridge early in the evening before officials decided to cancel a plan to use 60,000 gallons of water to flush away spilled chemical around the ruptured car.

The chemical, phosphorus trichloride, was in liquid form in the tank car, but vaporized into the acids when it mixed with air and moisture.

"Rather than flood it and run the risk of another gas cloud," said Paul Cahill, state deputy Civil Defense director, "it was decided . . . to cover the spilled chemical with dirt."

The chemical that remained in the damage tank car was pumped to tanker trucks.

The acid clouds also wafted high over Boston, but officials said there was no immediate threat to the city. However, several office buildings shut down air-circulation systems as a precaution.