Weekend BART riders, get ready. The transit system plans to halt service to work on its rails between the Lake Merritt and Fruitvale stations Saturday and Sunday, and will do it again on five more weekends through early July.

The shutdowns eventually will bring smoother, quieter and more reliable rides, BART officials promise. But during the six weekends of work, they’ll inconvenience 18,000 to 20,000 passengers on the Warm Springs-Richmond and Dublin/Pleasanton-Daly City lines as well as riders headed to or from Oakland International Airport.

BART will provide bus bridges where the trains are shut down and advises passengers to allow an extra 25 to 40 minutes for their trips.

While the tracks are closed, crews will work around the clock to replace interlocking, X-shaped crossover tracks that allow trains to change directions, as well as rotting wooden ties and worn rail.

The work is part of a continuing BART rejuvenation effort started two years ago, but it’s the first big project funded with money from Measure RR, the $3.5 billion bond measure to rebuild BART that voters approved in November.

“It’s the same kind of work we’ve done near Balboa Park, Fruitvale, Coliseum and West Oakland over the past couple of years,” said Taylor Huckaby, a BART spokesman.

To accommodate riders this weekend, BART will offer a free main shuttle service between 19th Street Station in downtown Oakland and Fruitvale Station. Riders are advised to expect trips to take 20 to 40 minutes longer. Signs and extra BART staffers will direct passengers between trains and buses.

Lake Merritt Station will be closed during the track work, but a free bus bridge will operate to and from Fruitvale Station. It is expected to add 10 to 25 minutes to trips.

BART service starts at 6 a.m. on Saturdays and 8 a.m. Sundays and the last trains leave end-of-the-line stations around midnight. The Fruitvale-Lake Merritt shutdown will last all day Saturday and Sunday. Trains will resume regular service when BART opens at 4 a.m. Monday.

Though the rail replacement work is becoming somewhat familiar, BART workers plan to do something new on this job to save time. They’ve built parts of the crossover tracks in advance and will hoist them into place using cranes, instead of building them in place.

More closures and service interruptions between Lake Merritt and Fruitvale are scheduled on the weekends of April 29-30, May 13-14, May 27-29, June 10-11 and July 1-2.

Michael Cabanatuan is a San Francisco Chronicle staff writer. Email: mcabanatuan@sfchronicle.com