Cambridge, Mass. — ''Hey, a worker left his glove on the floor here.'' A commuter who stoops to pick up the glove discovers that this is not a glove; it's part of the glistening tile floor at the new Porter Square station on the Red Line of the Massachusetts Bay Transportation Authority (MBTA). This is a feature of ''Arts on the Line,'' a program designed to make MBTA stations more eye-appealing for busy riders while they wait for the next train. Porter Square and Davis Square in Somerville are the newest stations to open in a $574 million project to extend the Red Line 3.2 miles northwest of Harvard Square.
One more station, Alewife, remains to be completed. It is scheduled to open next April, says MBTA spokesman Listo Fisher.
Art and elaborate new stations aside, better service and improved business are the basic reasons for the extension of the Red Line. When the northward push to Arlington is completed, the line will take riders from Alewife southeast to Braintree in south suburbia or to Mattapan Square at the southern tip of the Boston city limits.
''When we look at the growth of public transportation, we see the great economic development of Boston and vicinity,'' Gov. Michael S. Dukakis told big crowds Dec. 8 at grand openings of the two stations. ''With the expansion of public transportation we see increased local community business activity.'' The two new stations are sure to help businesses around them, the governer said and smiled. The project, initiated during his first term in office, is nearing completion. ''This new service gives both cities (Cambridge and Somerville) and the entire region a stronger transit network and a new foundation for economic growth,'' Mr. Dukakis concluded.
Other speakers - Mayor Eugene Brune of Somerville, Mayor Leonard Russell of Cambridge, and City Manager Robert W. Healy of Cambridge - praised the openings as a sign of the economic strength of neighborhood business centers. The new stations create more people traffic for firms located in the surrounding commercial areas, they insisted.
''We are relieved and pleased to see the Porter Square station open,'' says Davis Hughes, executive vice-president of the Cambridge Chamber of Commerce. ''We're anxious to see Alewife completed now.''
During the days of construction the MBTA subsidized a special project, ''Merchants on the Line,'' to help promote business for areas severely inconvenienced, Mr. Hughes recalls. That helped many of the small businesses survive, he says.
''And we got a bonus,'' he adds. ''Anticipating improved opportunity with completion of the project, homeowners and merchants improved their properties, especially along Massachusetts Avenue.''
The new Red Line makes Porter Square accessible to more people, Hughes says: ''We cut down on the traffic jams, and we have some new, improved business in place of rundown and vacant storefronts.''
''Davis Station is the cornerstone of the revitalization of the Davis Square community,'' Mayor Brune said. ''It boosts economic development of the business community. It preserves, it doesn't destroy, our neighborhood.''
The city's business community views the Davis station with ''cautious optimism,'' says Frank Stellato, executive director of the Somerville Chamber of Commerce. ''We hope this means more people will stop and do business at Davis Square, not pass us by to go elsewhere to shop.''
Praising the community's attractions - he lists the offbeat Somerville Theatre and the varied restaurants as ''special'' - Mr. Stellato echoes an advertising slogan of the past, ''The city has improved the atmosphere of the square, too. Try us. I know you'll like us.''
Both new stations are more than train stops in Somerville and Cambridge. They include passenger-oriented features such as:
Escalators. Not an unusual T station feature, but essential at these stations , which are far below street level - 140 feet to the Porter Square loading platform, somewhat less at Davis Square. From bottom to top, the Porter escalator takes 1:07 minutes for the trip.
Elevators. Also not unusual, but again a definite need in these stations for people who might find it difficult to use stairs or escalators to get to and from the loading platforms.
The usual in-station services. When the stations are completed, the MBTA will authorize concessionaires to sell newspapers, sundries, snacks, and other items.
Accessibility. Riders can catch commuter trains to the northwest from Porter Square.
And art. Through the MBTA ''Arts on the Line'' program, $500,000 worth of artworks have been placed in the new stations.
Porter Square spotlights unusual art forms such as the workers' gloves (in bronze by sculptor Megs Harries) on the floor surface, a giant collection of offbeat creations by sculptor Susumu Shingu hanging from the ceiling of an atrium-type open area in the giant depot.
This station also features large panels of black-and-white drawings of old railroad trains and train depots of the past.
Schoolchildren provided much of the decor of the Davis station. Their ''in the wall'' paintings are featured at numerous points along the tile station walls. Each work has the child artist's name imprinted on it. The Somerville Neighbronhood Task Force worked out this idea.