A new branch of the MBTA’s Silver Line began service on Saturday, connecting Chelsea to South Station with stops at Logan Airport and the Seaport Area. The new route appears on T maps along with the Red, Orange, Blue and Green lines. But if you’re expecting to see a subway or trolley, look again: It’s a bus.

Make that the fanciest buses in the T’s system. All Silver Line buses are big, articulated 60-footers painted silver and blue. Yet like the Green Line, there are different branches that aren’t all connected to each other. Routes SL1, SL2 and SL3 all start at South Station, with the SL1 going to Logan Airport, the SL2 to the Seaport Area, and the new SL3 to Chelsea. The SL4 and SL5 start at Dudley Square and travel down Washington Street, with the SL4 heading to South Station and the SL5 to Downtown Crossing.

How all those buses got to be is a bit of Boston transit history. Stick with me here.

The original Silver Line route, SL4, began as a replacement for the old Orange Line El that used to travel above Washington Street. When the train line was moved in 1987 to its present location a half-mile away, many Roxbury residents called for a streetcar to replace the train.

That led to a years-long battle with the T, which ended the discussion in 1998 by proposing the Silver Line as bus rapid transit, or BRT. The transit concept has been gaining popularity around the world, with a shining example in Guangzhou, China, where the system has earned “gold standard” BRT status from the Institute for Transportation and Development Policy.

Gold standard BRT is nearly as efficient as a train, and have three major characteristics: Buses run on a dedicated busway, in which other vehicles are banned; bus stops have raised platforms that allow wheelchairs and strollers to roll right on; and passengers pay their fares in stations rather than on the bus to speed up boarding.

With its new fare system approved last fall, the T is planning for riders to pay before boarding on all its buses and trains. But that’s still several years off, and on the Silver Line, riders have to pay at the front door. Silver Line buses have lower floors but no raised station platforms. And while there are dedicated bus lanes for portions of the routes, Silver Line buses have to negotiate through Boston traffic for the rest of their trips.

That’s a problem, said Mary Skelton Roberts of the Barr Foundation. She is co-director of climate for the Boston-based foundation that’s pushing for gold-standard BRT locally, but not necessarily along existing Silver Line routes.

“If you’re going to have gold standard BRT, most of it does need to be on a dedicated lane,” she said. “If it doesn’t, then you’ve created BRT in some spots, then you go back to the bus being in traffic and everything’s backed up and it almost defeats the purpose.”

In Chelsea, the new line will run on a bus-only road next to the commuter rail tracks and an old railroad spur for most of the trip through the first-string suburb. Mauricio Rojas lives nearby and is excited about the route.

“For the neighborhood, it’s the best,” he said. “We can go to the airport and South Station just in one ride.”

But that ride is interrupted when the bus merges into traffic just before the Chelsea River bridge, which rises for boats as many as eight times a day. When the bridge is up, the bridge operator can send a signal to the buses, which are outfitted with pre-recorded announcements for every scenario, such as, “This bus is being detoured while a drawbridge is raised on our normal route” and “We will not be stopping at airport. Next stop: Silver Line Way.”

So while riders may be delayed by the bridge, the buses will still get through.

Is the T's newest line a gold standard BRT? Not quite. But by the T’s definition, it’s a silver one.