Commuters on public transportation are given to venting outrage when their train or bus line raises fares, curtails service or both. So it's no surprise that commuters on the Baltimore-Washington rail line, operated over the years by the Pennsylvania Railroad/Penn Central/Con-rail/Amtrak, are outraged at the latest commuter-fare gambit.

In the starkest terms, Amtrak has announced that it will charge some--not all--commuters $9.20 between the two cities for a one-way trip, compared with $2.25 on a normal commuter journey. But before you, too, become outraged, consider the background:

Traditionally, the railroad charged single-trip passengers one fare and granted a reduced fare to commuters. But when the railroad threatened several years back to abandon the local commuter trains because of high costs, the state of Maryland stepped in and agreed to subsidize three weekday trains that carry about 500 riders in each direction, and to sell low-cost tickets intended primarily for use on them. But the discount tickets, which cost $99 a month for daily round trips between Baltimore and Washington (which works out to $2.25 a trip) also have been accepted on Amtrak's New York through trains.

Many people who miss the last homebound train at night or find Amtrak trains more convenient or faster than than the state-subsidized locals have continued to make use of the through trains.

That's coming to an end on April 1. Amtrak this week announced that as of that date the $2.25 commuter tickets will be accepted only on the state-subsidized locals, and it will demand $6.95 additional in cash (to equal the regular one-way Baltimore-Washington fare of $9.20) on the through trains. Travel between intermediate stations will cost proportionately less.

Aggrieved commuters complained. Stan Meiberg, president of the Association of Maryland Amtrak Commuters, met earlier this week with aides of Sen. Paul S. Sarbanes (D-Md.) in an attempt to persuade Amtrak to back off. But Amtrak spokesman John Jacobsen defended the move as necessary to play fair with non-subsidized passengers, as well as to produce revenue.

James A. Emery, director of passenger services for the Maryland State Railroad Administration, said that "we disapprove of the steep increase , but have no control over it" under the state's subsidy contract with Amtrak.

One problem faced by the state, Emery said, is that 200 to 300 passengers using state-discount tickets daily ride each way on the through trains, which may require an increase in the state-subsidized service.

Service over the more-or-less parallel Baltimore & Ohio Railroad route is not affected by the dispute.