The first official weekend of summer will come without BART train service for some who catch their trains north of Walnut Creek.

Busses will be the only way between the Concord and Walnut Creek stations on Saturday and Sunday morning while workers replace decades-old track components, the agency said. All train service between Concord and Walnut Creek will be halted to make the conditions safe for the workers.

The project also will bring the cancellation of some late-night and early morning trains, and riders in the area can expect delays of 20 to 30 minutes. The last southbound trains departing Antioch at 11:42 p.m. and Pittsburg/Bay Point at 12:10 a.m. both will be cancelled on Saturday and Sunday. They instead will depart at 11:23 p.m. and 11:44, respectively.

On Saturday, BART will run its first southbound Yellow Line trains on the system starting at 5:43 a.m. from the Walnut Creek station. Trains also are scheduled to depart Walnut Creek at 6:03 and 6:23. On Sunday, southbound trains will begin at 7:43 a.m. from the Walnut Creek station and depart every 20 minutes.

Buses will be available at the Concord station on Saturday beginning at 5:23 a.m. and at the Pleasant Hill station beginning at 5:30 a.m. The busses will leave those stations every 20 minutes and will drop off passengers at the Walnut Creek station. On Sunday, the busses leave Concord starting at 7:23 a.m. and Pleasant Hill at 7:30 a.m.