The on-scene phase of the investigation generally lasts between three to five days, NTSB spokesman Peter Knudson said Sunday afternoon.
The investigators, who are based in Los Angeles, are interviewing witnesses, passengers and the drivers of the trains. Other things that will be considered are the condition of the train cars involved, the signal systems and the condition of the tracks, according to Knudson.
NTSB officials were planning on having a news conference this afternoon in San Francisco to provide more specific details regarding their investigation.
The light rail vehicles resumed service at the West Portal Station about five hours after a train rear-ended another light rail vehicle at the station.
The collision occurred just before 3 p.m. when a L-Taraval train rear-ended a K-Ingleside train that was stopped on the outbound platform of the West Portal Station, Muni spokesman Judson True said.
Several people, including the operator of the L-Taraval train, suffered serious injuries. At least 40 others were transported to hospitals with minor-to-moderate injuries, True said. No deaths were reported.
The people with minor injuries were transported to area hospitals in a Muni bus, a procedure standard for emergencies, according to San Francisco fire Lt. Ken Smith.
Nearly 70 personnel from the San Francisco Fire Department responded, as well as 15 ambulances, seven engines and one fire truck, Smith said.
The collision halted service at the West Portal Station until about 8 p.m. when the K, L and M lines began running again.