After four years on a bureaucratic sidetrack, plans for modernizing and doubling the capacity to two railroad commuter lines in suburban Maryland have received a green light from the federal government.

The U.S. Department of Transportation has announced that it will grant $11.8 million for new equipment and station renovations on the Baltimore & Ohio Railroad lines from Brunswick and Baltimore into Washington's Union Station.

The lines serve a fast-growing area of upper Montgomery County and a densely populated corridor in western Prince George's County.

Jay Hierholzer, who heads the commuter rail program for the Maryland Transportation Department, said the first new equipment should be put into service about a year from now.

More than 5,000 people now ride state-subsidized trains on the two lines, a figure that surged after the opening of the first Metro subway line in 1976 made it easy to get from Union Station to downtown Washington. Aisles crowded with standees are now commonplace on the commuter trains.

Although the new federal grant will not quite double the number of cars in service, from 22 to 35, the new cars will have more seats, doubling the carrying capacity, Hierholzer said.

Maryland first sought the federal funds in 1974, but the program was snagged on labor negotiations required by federal law and later on newly imposed requirements that special facilities be provided for elderly and handicapped riders.

Both problems were resolved, Hierholzer said, and some of the new railroad cars will be fitted with devices to lift wheelchairs on and off station loading platforms.

In addition to improvements at 11 stations, which will include the enlargement of some parking lots, the program includes money to prepare plaus for a future new station at Silver Spring.

If that is eventually built, it will permit trains from Brunswick to transfer downtown-bound passengers directly onto Metro trains. The trains will then be able to go back and pick up another load of commuters during the same rush-hour period, further increasing the carrying capacity of the line, Hierholzer said.

In addition, a new station will be constructed at Metropolitan Grove Road northwest of Gaithersburg. It will have a parking lot for at lest 200 automobiles.

The Maryland General Assembly and the Montgomery and Prince George's governments already have appropriated nearly $3 million as a 20 percent share of the total $15 million cost of the program.

Hierbolzer said plans call for buying and refurbishing 25 conventional railroad coaches, 10 self-propelled, diesel-powered coaches, and 5 diesel locomotives. Some antiquated equipment now being used on the lines will be retired.

Stations to be improved are Laurel and Riverdale in Prince George's County, Elkridge and Jessup in Howard County, Brunswick in Frederick County, and Dickerson, Barnesville, Germantown, Gaithersburg, Garrett Park and Kensington in Montgomery County.