(Adds background on other crashes, details on critical NTSB report)
Feb 5 (Reuters) - A locked track switch was blamed for the collision of an Amtrak passenger train with a freight train that killed two people and injured over 100 in South Carolina on Sunday, raising questions about the delayed roll out of a system to prevent such crashes.
The head of the National Transportation Safety Board said a switch on the tracks, which the freight hauler CSX Corp owns and operates, was padlocked in a position that steered the Amtrak train onto a siding near state capital Columbia, in central South Carolina, where it crashed into a parked, unoccupied CSX train.
"Key to this investigation is learning why the switch was lined that way," Robert Sumwalt, the chairman of the NTSB, told reporters on Sunday. The NTSB plans an update on Monday on Amtrak's fourth deadly crash since December.
In November, the NTSB criticized what it termed Amtrak's "weak safety culture" after finding that a series of unsafe conditions led to an Amtrak train striking a backhoe working on railroad tracks in Chester, Pennsylvania, in 2016, killing two maintenance workers and injuring 41.
Amtrak President and Chief Executive Richard Anderson told reporters on Sunday that CSX was responsible for the wreck of Train 91 because of the locked switch. CSX officials were not available for comment.
Two patients remained in critical condition and another two were in "serious" condition at Palmetto Health hospitals, a spokeswoman said Monday.
Two of the recent Amtrak crashes involved vehicles driving around gates and being struck by trains, according to Anderson and North Carolina police. One of those crashes occurred last week when a chartered train carrying U.S. Republican lawmakers struck a garbage truck in Virginia.
Anderson said the South Carolina accident would have been prevented by positive train control, or PTC, a system designed to prevent derailments or crashes caused by excessive speed. Investigators say many deadly U.S. train crashes could have been prevented if the system was in place.
Last month, U.S. Transportation Secretary Elaine Chao urged U.S. railroads and transit agencies to take all possible measures to meet deadlines to install PTC.
"It is important to understand the factors that contributed to this tragic accident and how all stakeholders can ensure a safe and reliable rail system going forward," the department said in a statement Sunday.
A deadly Amtrak crash in December near Seattle that killed three occurred on a section of track that did not have PTC operating and could have prevented the derailment, the NTSB said. The NTSB also said the Amtrak engineer on that train told investigators he misread a signal shortly before the incident.
In 2008, Congress mandated the implementation of PTC nationwide by the end of 2015, then extended that deadline until the end of 2018.
(Reporting by Rich McKay, David Shepardson in Washington and Jonathan Allen in New York editing by Tom Brown and Andrew Hay)