Maryland wants to boost fares by 30 percent on state-subsidized commuter rail lines between Washington, Baltimore and Brunswick, near Harper's Ferry, W. Va., beginning in July.The trains carry about 5,000 commuters daily.

The fare increases would mean commuters would pay half the $8 million-a-year state subsidy of weekday commuter service on Amtrak, Conrail and Baltimore & Ohio Railroad trains. The shifting of costs to the riders is part of legislation signed by Gov. Harry Hughes last week. The new law will keep the commuter trains running in the state for at least three more years.

Because of dwindling revenues and the difficulty of finding enough railroad coaches, Hughes last summer proposed significant cuts in the state subsidy of passenger rail transportation. He proposed elimination of the daily commuter trains operated by the B&O between Brunswick and Washington, and of the Conrail commuter trains, which now carry two-thirds of the passengers between Baltimore and Washington.

Under the new legislation passed by the General Assembly, service will be continued on the two Conrail and B&O Brunswick lines as well as the B&O line that runs between Baltimore and Washington and the Chesapeake train, operated by Amtrak between Philadelphia and Washington.

The 5,000 commuters who ride the rails in Maryland -- including commuters on two other Amtrak trains scheduled to be eliminated in October, the Shenandoah and Blue Ridge -- are a relatively small but fast-growing group. Ridership on several lines has increased more than 25 percent in each of the past two years.

The fare increases on Conrail and B&O trains, which follow 10 percent fare increases last summer, must be approved by the Interstate Commerce Commission. The ICC is expected to approve some increase, state officials say, but may not approve the full 30 percent boost necessary to meet the state legislature's stipulation that passengers pay at least 50 percent of the operating costs of commuter rail service. If a lesser increase is approved, Maryland immediately will reapply for an additional fare increase, a spokeswoman for the State Railroad Administration said this week.

While steep fare hikes may be the bad news, the good news is that most commuter trains will continue to run in the state, and there will soon be newer and larger coaches on most of them.

Under a $17.5 million rail-improvement program, 80 percent of which is supported by a federal mass-transit grant, Maryland now is completing renovation of 22 railroad coaches and recently finished restoring 10 self-propelled diesel cars.

The project also covers $1.4 million worth of improvements made to railroad stations in Dickerson, Barnesville, Germantown, Gaithersburg, Garrett Park and Kensington, and renovation of the Laurel and Riverdale stations. Most of that work is due to be completed this year.

This investment by the state and federal governments in passenger trains and stations in Maryland apparently was one reason the legislature insisted the commuter rail programs be continued at present or "expanded" levels. At the moment, however, the state has no plans to increase the number of commuter trains.

The General Assembly also prodded state officials to reopen negotiations with New Jersey over leases of 10 railroad cars used by Conrail on its Baltimore-Washington commuter line, the line Hughes proposed eliminating.

New Jersey had said it wanted the 10 cars back, perhaps as early as this summer, but since has agreed to lease to Maryland 10 other passenger cars. The new lease carries a rate of $75,000 a month, compared with the present $60,000. The new cars will hold more passengers -- about 120, compared with a capacity of 98 on the older cars.

The Conrail line has been operating at full capacity or more.Both the Baltimore and Ohio Railroad commuter lines between Brunswick and Washington and Baltimore and Washington have been operating at an average 108 percent of capacity, according to state Railroad Administrator Charles H. Smith.

How long the present commuter rail arrangement will last is unclear. Federal funds will pay one-fourth of the cost at least through 1982, but Smith said the Reagan administration's proposal to phase out federal mass-transit subsidies of this kind could mean the end of all federal aid by 1984. The New Jersey railroad car lease is expected to run at least through the end of 1982, when that state expects it will need the trains