The Reagan administration's proposed budget cuts, rising costs and Washington Metro subway construction threaten the traditional commuter trains that carry 3,800 riders a day to work in Washington.

While their number is a minuscule percentage of the more than 1 million people who find their way to work in the metropolitan area every day, their problem is similar to that faced by the thousands of people who ride such trains to work from the far-flung suburbs of New York, Philadelphia and Chicago:

They purchased homes many miles from their jobs and assumed they could ride the New Haven or the Illinois Cnetral or the B & O to work. The Reagan administration, for whom many of them voted, now proposes to cut the federal aid that in many cases keeps their trains running.

In short, the situation poses the classic questions of transportation economics. Who pays? How much cost should be borne by the user? How much is in the public interest and thus eligible for general taxpayer support?

The total federal and state subsidy for traditional, commuter-carrying railroads in Maryland exceeds $5 million. Should passenger fares be raised to cover the federal $3.2 million and the state's $2 million? If that happens, will riders join the herd on the freeway?

These are not easy questions, and as Congress ponders them members will find that the railroad riders of the Washington area are unusually skilled lobbyists; many of them work on Capitol Hill.

Here is the situation:

The Maryland Department of Traqnsportation sponsors three commuter railroad lines that carry about 3,100 people a day. Two of the lines run between Baltimore and Washington. Trains on the third run between Washington and Brunswick, a Potomac River railroad town about 45 miles from Washington, with stops in Silver Spring, Rockville, Gaithersburg and Point of Rocks, among other places. Federal aid for these trains totals $2 million, and it the Reagan proposals survive in Congress, will be eliminated in 1985. "We don't know what we're going to do about the loss of that aid," said Jay Hierholzer of the state railroad administration.

Amtrak runs two trains, the Shenandoah and the Blue Ridge, that carry 700 commuters a day between Harpers Ferry and Washington. Both trains will die Oct. 1, along with a lot of other Amtrak service, unless Congress changes the Reagan administration's proposed cuts in Amtrak appropriations. The Blue Ridge, a train run strictly for commuters, requires a federal subsidy of $1.2 million. The Shenandoah, a long-haul train from Washington to Cincinnati, runs an annual loss of $4 million, an unknown amount of which results from serving Washington commuters.

Metro's Orange Line from New Carrolton runs parallel to part of one of the Maryland lines between Baltimore and Washington. Metro's Red Line to Shady Grove in Montgomery County, due to open late in 1983, will intersect the Washington-Brunswick line. The Maryland Department of Transportation, which subsidizes Metro as well as the commuter railroads, would like to eliminate the Baltimore-Washington line that parallels the Orange Line and is studying termination of the Brunswick line at either Shady Grove or Silver Spring instead of Union Station when Metro's Red Line is extended.

The Maryland legislature, responding to skillful lobbying from cummuters, blocked that proposal in the most recent session, but the idea is certain to reappear because it would save the state's hard-pressed transportation fund substantial money.

While that saves taxpayers money, it would be inconvenient at best for some commuter train passengers. If one of the Baltimore-Washington lines is eliminated, its riders would have to drive to New Carrollton and catch Metro; drive to the other Baltimore-Washington line, which is full, or just drive.

If the Brunswick trains are terminated at Shady Grove, those riders who work in Silver Spring, not Washington, will have a very long, expensive subway ride. They are anxious to prevent that.

Complicating the question for Maryland about whether to continue subsidizing the trains is the fact that at least 1 passenger in 5 on the Brunswick line lives in West Virginia or Virginia and drives across the Potomac River to Maryland to catch the train. Maryland taxpayers are paying the cost of the Loudoun County gentleman farmer's ride to work, but getting none of his tax money.

The same equity problem arises if Maryland were to extend its Brunswick trains to Harpers Ferry in the event the Amtrak trains are eliminated.

West Virginia Gov. Jay Rockefeller, according to Hierholzer, included money for the Maryland trains in his most recent budget, but then withdrew it when Maryland DOT announced plans to terminate someday the Brunswick trains at the future Shady Grove Metro station.

The state of Virginia, as a matter of longstanding philosophy, refuses to subsidize forms of transportation that do not pay a gasoline tax.

Many commuter railroad riders work on Capitol Hill and know their way around the legislative process, which help explains their recent success with the Maryland legislature. Its members were hit with a blizzard of well-reasoned, carefully prepared arguments.

"Raise our fares," the commuters said, "but save our trains."

Gov. Harry Hughes last week signed the bill requiring all present commuter service to continue, and Marylnad is now seeking approval from the Interstate Commerce Commission for a 30 percent fare increase.

Maryland has done much in recent years (with the aid of a federal grant) to improve the trains it is running. New or rebuilt cars have been or soon will be placed on all the lines and stations have been upgraded. But the work has emphasized rehabilitation, not added capacity.

That leaves the question of Amtrak, and here again the commuters are well organized. Take, for example, Terry Flaherty.

Flaherty is the immediate past president of Friends of the Railroad. He lives in Harpers Ferry and rides a train to Washington every day, where he is the director of the Senate Parking Garage. Senators park their cars there, and if they have a parking problem, they take it up with Flaherty.

Now Terry Flaherty has a problem. Does he take this up with his friends in the Senate?

"I have to be pretty careful about that," Flaherty said, and he referred the caller to the new president of Friends of the Railroad, Jeffrey Esser.

Esser is the director of the federal liaison center for the Municipal Finance Officers Association, which means he is a lobbyist on Capitol Hill. He lives in Loudoun County, drives across the Potomac River to Maryland and boards the train to Union Station.

When it comes to fighting for the railroads, Esser said, "There's a lot of talent, but it doesn't mean we'll always win." Nonetheless, he acknowledged, "Our testimony may be a little more polished than from an average citizens group."

Esser and Flaherty will need all the polish they can muster to save the Amtrak trains to Harpers Ferry. The Shenandoah, a cross-country train between Washington and Cincinnati, is on the critical list because it does not even meet Amtrak's minimum requirements to stay in service.

The Blue Ridge, however, is a different case. It is one of several short-haul trains around the country that Congress designated for commuter use, and some congressional support is building to add $10 million to the Reagan budget and keep them alive for another year.

There are no commuter trains in Virginia. When the state Department of Highways and Transportation was asked if such service had ever been considered, a planner found the sole surviving copy of a 1974 study entitled "Commuter Railroad Feasibility in Virginia," and lent it to a reporter.

The study concluded, in part, "that commuter rail passenger service is feasible in Northern Virginia but there are critical issues to be resolved before implementation is possible."

Richard C. Lockwood, of the state transportation department, said in an interview that "there is not much hope commuter rail will be pursued actively." i

Chiseled in stone over the main entrance in Richmond to the state Department of Highways and Transportation are these words:

"Dedicated to the comfort and safety of those who travel the highways of the Commonwealth of Virginia."