Firefighters from the San Bernardino County Fire Department, Cal Fire and the U.S. Forest Service responded to the scene near Cleghorn Road shortly before 1 p.m.
San Bernardino National Forest officials announced the closure of all lanes on 15 Freeway at about 2 p.m. Less than an hour later, California Highway Patrol began escorting southbound traffic through the area while the northbound side of the freeway remained shut down.
By 3 p.m., all northbound lanes reopened, except for one which remained closed, according to Caltrans.
The forward progress of the so-called Railroad Fire was stopped after it burned about 25 acres, officials said. No injuries or structural damage were immediately reported.
The cause of the blaze is under investigation.