A federal audit finds that the nation's railroads are overwhelmingly behind on implementing positive train control, a safety technology experts say would have averted a deadly Amtrak derailment near Philadelphia this year that killed eight people and injured 200 others.

As many as 70 percent of rail operators say they will fail to meet a key year-end deadline for deploying positive train control (PTC) on their routes, according to the Government Accountability Office. Some will miss the target by a year; others, as much as five years.

Only five railroads — SEPTA in Pennsylvania; Silicon Valley's Caltrain system; Southern California's Metrolink; Portland's TriMet; and Portland and Western Railroad — will successfully meet a legal requirement to install PTC by Dec. 31.

Positive train control works by regulating the speed of a train based on its location and the data it receives from PTC equipment embedded in the tracks or mounted nearby. Information about signal status and local speed limits can be relayed through PTC to the locomotive, which then automatically applies the brakes if the train is moving too quickly.

In the case of the Philadelphia crash, Amtrak train 188 leapt off the rails after barreling into a curve at 102 miles per hour, over twice the speed limit. Investigators said that in the moments leading up to the crash, the locomotive actually accelerated, for unexplained reasons. Had positive train control been active, the safety feature would likely have intervened, saving lives.

Even the few railroads that report being on track with PTC may face problems using the technology, GAO said.

"The ability of these five railroads to achieve full PTC implementation may be affected because other railroads they interact with may not be equipped by the deadline," the agency said in its report Wednesday. In addition, the complex business relationships between companies that all use the same tracks could pose a roadblock to progress.