By David Morgan

WASHINGTON (Reuters) - Railroad companies that fail to meet a Dec. 31 deadline for installing new safety equipment could face U.S. government penalties, federal regulators told Congress on Tuesday, in the wake of last month's deadly Amtrak passenger train crash in Philadelphia.

The pledge of action, which could include daily civil penalties, showed Washington was exerting renewed pressure on train operators to adopt safety technology that could have prevented the May 12 derailment.

Nonetheless, some lawmakers were frustrated by the pace of the investigation.

"There are very few answers right now, three weeks after one of the most horrific crashes that our nation has ever seen," said Republican Representative Jeff Denham, of California.

Sarah Feinberg, acting administrator of the Federal Railroad Administration, told a congressional panel: "FRA feels strongly that the deadline of December 31, 2015, is an important mandate for the implementation of PTC (Positive Train Control) and our agency intends to enforce it."

Amtrak is expected to meet that deadline, but other train operators, such as some commuter and freight lines, may not, Feinberg told the House of Representatives committee.

PTC automatically slows trains that reach unsafe speeds, reducing the chances of human error, which remains a strong focus of a National Transportation Safety Board inquiry into the crash that killed eight people and injured about 200.

Investigators are also looking into the conduct of the train engineer as the train hit speeds of 106 miles per hour (171 km per hour) in a zone with a 50-mph (80-kph) speed limit.

NTSB Chairman Christopher Hart testified at the House Transportation and Infrastructure Committee's hearing that PTC would have prevented the Philadelphia derailment.

Feinberg said her agency wants Congress to help it impose interim safety measures for train operators that miss the deadline for installing PTC.

She warned of slapping penalties on train companies, but added, "some railroads have behaved better than others and we don't want to punish those who are farther ahead than those who've not done anything at all." She did not specify which companies were lagging.

While the House committee members pressed for quick installation of the safety equipment, there is bipartisan support in the U.S. Senate for delaying the PTC requirement until at least 2020.

It was not yet clear whether legislation along these lines will be sent to President Barack Obama or whether he would sign it into law.

Representative Bill Shuster, the Pennsylvania Republican who chairs the committee, said at the start of the hearing: "We are not aware of any defects or issues identified to date with respect to the track, locomotive or other infrastructure."

Federal officials told the panel they do not yet know whether a "dead man" safety switch was operating before the derailment.

Meanwhile, Obama issued a veto threat on Tuesday to a bill proposed in the Republican-controlled House that would set Amtrak funding at $1.14 billion next year, or $262 million below the current level.

Obama's 2016 budget requested $2.45 billion for rail services to, among other things, "help bring Amtrak's Northeast Corridor infrastructure and equipment into a state of good repair."

The House bill also denies additional funding for safety inspectors.

Some lawmakers expressed frustration about a lack of information on cellphone records of 32-year-old engineer Brandon Bostian. Bostian's attorney Robert Goggin has said Bostian turned off his cellphone, as Amtrak requires of engineers behind the controls.

(Additional reporting by Susan Cornwell, writing by Richard Cowan; editing by Kevin Drawbaugh, Lisa Lambert and G Crosse)