During the closures, construction crews will be building crossover tracks that will allow trains to cross onto another track to avoid a disabled train ahead. The change is intended to reduce service delays.
The crossover tracks will also enable trains to turn around sooner and go in the opposite direction during commute times, which will be the equivalent of adding an extra train to the line, according to BART.
This weekend's closure is scheduled to begin at 9 p.m. Friday and continue through 1:30 a.m. Monday. BART service will resume at its normal time for the Monday morning commute.
The second closure is scheduled from May 28 to June 1.
BART is working with the Contra Costa County Transportation Authority to provide a free bus bridge between the Walnut Creek and Pleasant Hill stations during the closures, but passengers should allow up to 20 additional minutes for the detour.
One-third of the project's funding, about $13 million, comes from the American Recovery and Reinvestment Act of 2009.
The project, scheduled to wrap up this winter, is expected to create 1,200 jobs directly and indirectly and improve service for Pittsburg/Bay Point riders, according to BART.