(CNN) - Sunday's train collision in South Carolina was the fourth fatal incident involving an Amtrak train since the start of December.
The circumstances surrounding each crash vary; in two cases, vehicles appear to have driven around the lowered arm at a train crossing. The National Transportation Safety Board is investigating three of the collisions, including Sunday's.
Here is what we know about the fatal incidents:
Two Amtrak employees were killed when the train they were working on collided with a CSX freight train around 2:35 a.m. Sunday in Cayce, South Carolina, Amtrak officials said. The passenger train had been diverted off the main track onto a rail siding, where it crashed into a stationary freight train, a National Transportation Safety Board official said.
Local officials said 116 people were injured and taken to local hospitals. Eight crew members and 139 passengers were on the Miami, Florida-bound train, Amtrak said.
The victims were identified as train engineer Michael Kempf, 54, of Savannah, Georgia, and a conductor, Michael Cella, 36 of Orange Park, Florida, according to Lexington County Coroner Margaret Fisher.
Amtrak Train 91 was heading southbound and should have continued straight along the tracks, but the rail switch had been manually set to send the train onto the rail siding, where the CSX train was parked, NTSB Chairman Robert Sumwalt said Sunday.
"Key to that investigation is learning why that switch was lined that way," Sumwalt said.
Sumwalt said the crash could have been avoided if positive train control or PTC technology, which can automatically slow down a speeding train, had been in place.
Amtrak President and CEO Richard Anderson said the railroad is owned by CSX, which has complete control of the track, signals and switching where the crash occurred.
The CEO says the CSX dispatch center was manually directing the Amtrak train using telephone communications because the signal system along that section of the track was down.
An Amtrak train carrying Congressional members, including House Speaker Paul Ryan, to a Republican retreat in West Virginia struck a garbage truck near Charlottesville, Virginia on Wednesday, killing at least one person.
Investigators looking into the crash are focusing on the actions of the driver of the truck, a source with knowledge of the investigation told CNN.
Eyewitnesses have told NTSB investigators the truck driver was seen trying to snake his way through the crossing gates, despite signals that included lights warning of the oncoming train, two sources with knowledge of the investigation told CNN.
Anderson told reporters on Sunday that the garbage truck bypassed the crossing signals in the Virginia collision. He said Amtrak and transportation and rail officials have to invest more in technology to make the crossings safer and educate people about them.
"The issue you had last week was highlighted because there were many Congressmen on that train," he told reporters. "But I can tell you that happens almost every day somewhere in America ... and it has to do with people trying to drive around the arms that are down -- just like this garbage truck did -- and ignoring the bells and whistles that are telling you to stop because a train's coming."
"What I just want to reaffirm is that Amtrak is fully committed and values safety as its highest priority," Anderson added.
Eugene Lyons, a pastor, and his wife, Dorothy, were killed in Nash County, North Carolina when an Amtrak train hit their SUV on January 14, CNN affiliate WNCN reported. Both were in their 60s. It appears the SUV had driven around the lowered crossing arm, WNCN reported.
Whitakers police told the station a witness confirmed Dorothy Lyons was driving. Police didn't know then why she drove around the crossing arm.
The couple couldn't get out of the way in time to avoid the train, which was traveling about 55 miles per hour, the station reported. Both died at the scene.
Lyons' church stood about 200 yards from the train crossing, according to WNCN.
The Savannah, Georgia-bound train departed from New York City, Amtrak officials said.
The Amtrak Cascades Train 501 derailed near DuPont in Washington on December 18 and hurtled over an overpass onto Interstate 5, killing three people on its inaugural journey from Seattle, Washington to Portland, Oregon.
The locomotive engineer told the NTSB the train was traveling at about 80 miles per hour as it passed milepost 15.5 on the route, the NTSB said. He told the investigators he had planned to start braking about a mile before an upcoming curve with a 30-mph speed restriction at milepost 19.8, the agency said.
The engineer said he saw mileposts 16 and 17, but did not recall seeing milepost 18 or the 30-mph advance speed sign two miles before the curve, the NTSB said.
"The engineer said that he did see the wayside signal at milepost 19.8 (at the accident curve) but mistook it for another signal, which was north of the curve," according to the NTSB.
The engineer saw the 30-mph sign at the entrance to the curve and applied the brakes shortly before the train derailed, tumbling off the overpass as it entered the curve, the NTSB said.
A CNN investigation found that engineers and conductors had warned their supervisors that they did not feel adequately trained on the new route, according to more than a dozen sources.
The NTSB has said PTC was not yet functional on the locomotive.
Railroad safety experts have pushed for PTC, and Congress passed a law mandating it in 2008. Railroad companies have until the end of 2018 -- or, the railroads are granted an extension, until the end of 2020 -- to implement PTC.
Amtrak has equipped 49% of its locomotives and 67% of its tracks with PTC, according to Federal Railroad Administration data from the second quarter of 2017.