(CNN) - Railroad accident investigators are warning that further delays in installing crash prevention technology will continue to put travelers at risk of deadly accidents caused by human errors.
The National Transportation Board said at a safety event Monday that positive train control, or PTC, systems could have saved 303 lives and prevented 6,800 injuries over the last 50 years.
Congress passed a law in 2008 requiring rail operators to install the radio- and satellite-based technology by the end of 2015, but the railroad industry has repeatedly requested and received waivers for that deadline, citing expense and technological complications.
"There should be no more extensions on PTC," NTSB member Jennifer Homendy told CNN after the board named PTC one of its 10 most wanted reforms to improve transportation safety, alongside others such as eliminating drunk and distracted driving.
PTC prevents crashes and derailments by warning engineers and automatically applying the brakes to slow or stop trains that are too close to other trains or are traveling too fast.
It could have prevented an Amtrak train from slamming into an unmanned freight train, killing two and injuring 116. Two months earlier, another Amtrak train derailed, killing three and injuring more than 100, while speeding at 80 miles per hour through a 30 mph zone -- another preventable crash.
As the 2015 deadline approached, Congress extended the deadline to 2018. But the law also allowed railroads to request a waiver for as long as two additional years.
Only four railroads of 41 rail systems made the December 31 deadline, the Department of Transportation said in January. The other 34 have either requested or received waivers.
The Association of American Railroads, which represents large freight rail companies, said last month that PTC is fully installed across its entire fleet and tracks, and is operable on 83% of its route miles.
Passenger rail systems, which are generally less profitable than freight transportation, lagged behind in PTC implementation. The most recent data, from September 30, showed 26% of route miles were in operation.
The Federal Railroad Administration told CNN it was compiling updated numbers but did not have them available Monday afternoon.
John Hiatt, a former railroad engineer who is now an investigator at a law firm representing railroad employees, told CNN last month that railroads bear some responsibility for the missed deadlines, but that the blame lies with Congress for repeatedly extending the deadline.
"The blood is on their hands," Hiatt said of Congress.
Homendy, from the NTSB, said train travel is "generally" safe.
"It's one of the safest modes of transportation," she said. "But the risk of a PTC preventable accident is still there."
She acknowledged that the equipment, radio frequencies and staff training involved in PTC is expensive.
"But in our view," Homendy said, "there's a greater cost to losing a life."
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