Los Angeles ? The California agency responsible for train safety voted unanimously Thursday to direct railroad operators immediately to ban the use of cell phones by on-duty engineers, conductors and other rail workers.

Officials with the Public Utilities Commission said the deadly Metrolink crash last week was at least the third train accident in which the use of cell phones might have been a factor. Twenty-five people died and 135 were injured when a commuter train ran through a warning signal in Chatsworth, northwest of downtown Los Angeles, and collided head-on with a Union Pacific freight train.

Federal investigators said this week that records from Metrolink engineer Robert M. Sanchez’s cell phone show he sent and received text messages while on duty Friday. But they have not determined whether Sanchez was using his phone at the time of the accident.

“It’s extremely unfortunate that it took this terrible Metrolink-UP tragedy to focus our attention on this very serious issue,” said commission member Rachelle B. Chong.

Cell phone use would be allowed in emergencies but only after a train was stopped.

“The emphasis will be on making sure the owners of the trains are enforcing this rule,” said Richard Clark, a PUC safety director. “We will hold the owners of the trains responsible.”

Fines would start at $500 per violation, but could go as high as $20,000.

Officials had been concerned about cell phone use before last Friday’s crash because it appeared to be a factor in a June accident in San Francisco and another in July in Sacramento. Officials said they are depending on rail workers and members of the public to report violations and might set up a hot line to receive complaints.