Tijuana trolley; From San Diego to the border in 40 carefree minutes

San Francisco isn't the only California city with a transportation system that draws tourists - not since last July, when San Diego inaugurated its fire-engine-red trolley service. With an almost fire-engine-like speed - on an average under 40 minutes - it covers the 16-mile stretch of track from downtown to the Mexican border, carrying from 12,000 to 18,000 riders daily.

Of course, not all of them are tourists. San Diego's trolley is part of the city's regular public-transport network, and it's used by a great many commuters to the Navy yards, for instance, and to get from some of the bedroom suburbs into town. But since it began operation it also has drawn riders who want to add the fun of trolley riding to their San Diego visit, or who want an easy and convenient way to get to one of the city's major tourist attractions: Tijuana, just across the border in Baja California.

''I rode it,'' an enthusiastic friend said recently. ''I've lived in southern California all my life and have never even ridden a bus before. When I saw it, I knew I had to have a ride! It wasn't clanky or strident; it was cheap; and, in the long run, it was easier than crossing the border in my car.''

To cross the border in your own car may seem the easiest way to go, but it often entails a long, drawn-out customs search upon your return. In addition, few drivers would do it without an extra and sometimes costly insurance policy.

Another way to cross the border is via Mexicoach, a bus service that leaves San Diego's Santa Fe depot about every two hours and goes nonstop through San Ysidro to a terminal about a block from Tijuana's shopping district. The cost is customs checks - longer coming back.

San Diego's trolley cars don't themselves cross the border. They drop passengers at San Ysidro on the United States side. From there it's a zigzag walk across a longish ramp into Mexico. There is seldom any customs hassle for tourists on foot.

Buses and taxis wait on the other side to take you - for $1 or so - the mile-long journey to the stores on Avenida Revolucion.

It costs $1 to ride the trolley each way (40 cents for seniors and the handicapped). In town the best place for visitors to board is outside the picturesque Santa Fe railroad depot, though there are 17 other stations along the route (some with free parking and bicycle racks). There are bilingual staff people under a colorful umbrella outside the depot to explain the mechanics of the system and to hand out informational brochures and maps.

They will make change, if you need it, or show you how to pay your fare: It's on the honor system - no collectors or turnstiles. You buy a ticket from a machine at the station before boarding. Then you merely present it, if asked, to the roving ticket inspectors who make spot checks.

To board the trolley, press the lighted green button on the right of the door; the doors open and stairs appear. (A wheelchair lift is available on every train at the entrance nearest the operator.)

Then just sit back and watch the palms and pastel adobe houses and the US Seventh Fleet roll by.

The San Diego Light Rail Trolley (often called the ''Tijuana trolley'' because of its border destination) isn't by strict definition a trolley at all; it has neither a trolley wheel nor a pole connecting the cars to overhead wires. Instead, a scissorslike device, called a pantograph, powers the fleet of 14 Siemens-DuWag U2 vehicles along the track.

The bright red, German-manufactured cars give a somewhat European flavor to this very southern California community.

The trolley is the brainchild of Maureen O'Connor and Judith Bauer, chairwomen of the Metropolitan Transit Development Board (MTDB). It was funded with state monies derived from gasoline taxes. State Sen. James Mills spearheaded the legislation that set aside highway funds to be used for the rail development. San Diego Mayor Pete Wilson also was a very visible advocate of the

It looks as if the money was well spent: More people are riding the trolleys this first year than had been anticipated - about 30 percent more. In surveys conducted on board, riders questioned gave the trolley high marks for convenience, comfort, and speed.

And those riders are, in large part, helping the trolley pay for itself. ''Fare-evasion rate is very low,'' says a spokeswoman for the MTDB, ''just two-tenths of 1 percent; and farebox recovery - the percentage of operating costs covered by passenger fares - is above 75 percent. The national average is about 40 percent.''

Currently, the two-car trains run every 20 minutes in both directions from 5: 30 a.m. to 9:30 p.m. Next September, when construction has been completed on a full-length double track, trolley hours will be extended later into the evening.

Why a trolley line, instead of adding a few more buses? According to the MTDB , it's clean and energy efficient. Both are important factors in a part of the country known for its gasoline consumption and a growing smog problem.

And there's another plus: In car-crazy southern California, the very novelty of the trolley should attract riders - and help keep them.