Twenty-five people were killed and 135 injured in last week's commuter train accident in suburban Los Angeles.
The investigation is not finished, but the National Transportation Safety Board says the commuter train operator was using his cell phone before the head-on collision with a cargo train.
"Here's today's LA Times, 'Train engineer received text messages while on duty,'" said President Michael Peevey, of the PUC.
In San Francisco, the state PUC voted unanimously to make an emergency temporary rule banning train operators from using cell phones while trains are in motion. Peevey said the state had to act because the Federal Government had not.
"We've reviewed our rules and we've also looked at the rules of the Federal Railroad Administration and the Federal Transit Administration and found that there is no explicit rule prohibiting the use of a cell phone or electronic devices," said Peevey.
"I was astounded to hear that there was no rule and we would be remiss if we did not check with every other utility we regulate," said Dian Grueneich, a PUC commissioner.
The PUC says most, if not all rail and transit agencies already have similar rules in place.
"This gives us the authority to ensure that they enforce those rules and gives us the authority to assess penalties or halt operations if necessary," said PUC Customer Protection Director Rich Clark.
Clark says fines will range from $500 to $20,000 and be evaluated on a case-by-case basis.
Driver cell-phone use is a possible cause of a San Francisco Muni collision in June and the PUC says a Sacramento transit worker was killed in July in an accident involving a cell phone use.
The new PUC rule is now in effect.