Maryland's commuter rail line will increase service between Baltimore's Penn Station and Washington's Union Station next month by adding two daily round trips as part of a long-range drive to lure more passengers to the state-subsidized railroad.

The additional service, scheduled to start July 5, represents the first expansion of the Baltimore-Washington commuter line since 1976, when the Penn Central failed.

"We think it will be an attractive service to sell and attract new riders," said Charles H. Smith, administrator of the Maryland State Railroad Administration.

The move, approved Tuesday by the Maryland Board of Public Works, will increase the number of southbound trains from Baltimore to Washington during morning rush hours to four and will for the first time offer a northbound commuter train in the morning from Washington to Baltimore. During afternoon rush hours, the line will provide four northbound trains and one southbound train.

At present, commuter rail service between Baltimore and Washington includes three southbound trains in the morning and three northbound trains in the afternoon. Two of these round trips are provided by the Maryland rail line. The third has been offered under an agreement between Maryland and Pennsylvania. About 1,500 passengers use the commuter rail service daily.

Maryland's move to expand service was triggered partly by a recent decision by Pennsylvania officials to withdraw from the commuter rail accord. Under the pact, Maryland and Pennsylvania split the cost of running a train known as the Chesapeake between Philadelphia and Washington. Pennsylvania plans, however, to stop underwriting the train's costs, and Maryland concluded it could not finance the train without Pennsylvania's aid.

The Chesapeake is scheduled to stop running July 1. Maryland has decided to replace the Chesapeake with a new commuter train between Baltimore and Washington.

Maryland's plans for increased commuter rail service also stemmed partly from a controversy over fares. Passengers with low-cost commuter rail tickets have been allowed to ride on higher-cost Amtrak trains, leading to objections from Amtrak and a loss of passengers on the commuter line. Under the new plan, commuter tickets will no longer be accepted by Amtrak in rush hours.