The Metro-North workers were joined at the site by investigators from the NTSB, there to determine the cause of the accident.
They retrieved electronic devices like the cab video cameras and event data recorder.
After examining the damaged infrastructure closely, they took samples back to Highbridge for testing.
Later they interviewed the crew of the CSX train, the Metro-North engineer who had witnessed the derailment and notified the CSX crew, and Metro-North employees responsible for maintenance of way.

They found a wheel flange mark on the inside of the western rail at a point almost 73 feet (22 m) north of the control point where the CSX train had slowed almost to a stop when it saw the red.
This they identified as the point of derailment (POD).
They noticed immediately that the insulators on the Pandrol clips holding the rails to the concrete ties had slipped out of place slightly, with a noticeable gap between them and the rails in some areas.
At the POD the rail was bowed slightly outward, resulting in a ⁵⁄₁₆-inch (7.9 mm) variance from gauge.

At and near the POD the ballast and track were notably paler.
This was because they were covered with dust, indicating that soil infiltration had occurred.
The pale dust was not the soil itself, but residue from the tie ends, which had suffered serious abrasion.
Wheel flange grooves were also evident in some of the ties taken back to Highbridge, suggesting the track had been bending outward under load due to decreased structural support from the fouled ballast.
This evidence was noted in 2010 satellite photos of the area on Google Earth and more recent photos taken by Metro-North itself.


After making these measurements, they compared them with data from recent FRA and Metro-North track inspections.
In early June, the agency had gone through the area with its track geometry car.
At the POD it had recorded a 2-inch (51 mm) dip in the same track, consistent with motion noted on the cab video cameras in both CSX locomotives, and recorded the gauge as 0.06 inches (1.5 mm) over the regulatory standard of 57 ³⁄₄ inches (1,470 mm) for Class 2 track.
Both values were within tolerance.

The day before the accident, Metro-North had conducted a visual inspection of the track in the area on foot.
The railroad's inspectors noted no reportable defects at or near the POD.
However, within a mile (1.6 km) to the north of the accident, there were others, including broken ties and poorly drained areas.

Upon review of Metro-North's maintenance records, the investigators learned that the track and concrete ties had been installed in 2000.
The railroad's plans indicated that the track was to be resurfaced (i.e., the ballast replaced) every three years, and the ties and rails replaced every seven.
However, since a 2004 resurfacing, neither action had been taken.
Metro-North's chief engineer admitted the railroad was behind.
He could not explain why, but he said Metro-North was "working towards getting back into phase."

More recently, the NTSB noted, following a derailment of a passenger train in Connecticut two months earlier, which had also resulted from a defective and inadequately maintained rail, the railroad had hired Transportation Technology Center, Inc. (TTCI), to evaluate its track maintenance program.
Among the tasks undertaken by TTCI was to survey the entire Metro-North system for areas exhibiting surfacing issues like fouled ballast and poor drainage.
One of the 654 such areas it found was the POD.
The Metro-North workers were joined at the site by investigators from the NTSB, there to determine the cause of the accident. They retrieved electronic devices like the cab video cameras and event data recorder. After examining the damaged infrastructure closely, they took samples back to Highbridge for testing. Later they interviewed the crew of the CSX train, the Metro-North engineer who had witnessed the derailment and notified the CSX crew, and Metro-North employees responsible for maintenance of way.
They found a wheel flange mark on the inside of the western rail at a point almost 73 feet (22 m) north of the control point where the CSX train had slowed almost to a stop when it saw the red. This they identified as the point of derailment (POD). They noticed immediately that the insulators on the Pandrol clips holding the rails to the concrete ties had slipped out of place slightly, with a noticeable gap between them and the rails in some areas. At the POD the rail was bowed slightly outward, resulting in a ⁵⁄₁₆-inch (7.9 mm) variance from gauge.
At and near the POD the ballast and track were notably paler. This was because they were covered with dust, indicating that soil infiltration had occurred. The pale dust was not the soil itself, but residue from the tie ends, which had suffered serious abrasion. Wheel flange grooves were also evident in some of the ties taken back to Highbridge, suggesting the track had been bending outward under load due to decreased structural support from the fouled ballast. This evidence was noted in 2010 satellite photos of the area on Google Earth and more recent photos taken by Metro-North itself.

After making these measurements, they compared them with data from recent FRA and Metro-North track inspections. In early June, the agency had gone through the area with its track geometry car. At the POD it had recorded a 2-inch (51 mm) dip in the same track, consistent with motion noted on the cab video cameras in both CSX locomotives, and recorded the gauge as 0.06 inches (1.5 mm) over the regulatory standard of 57 ³⁄₄ inches (1,470 mm) for Class 2 track. Both values were within tolerance.
The day before the accident, Metro-North had conducted a visual inspection of the track in the area on foot. The railroad's inspectors noted no reportable defects at or near the POD. However, within a mile (1.6 km) to the north of the accident, there were others, including broken ties and poorly drained areas.
Upon review of Metro-North's maintenance records, the investigators learned that the track and concrete ties had been installed in 2000. The railroad's plans indicated that the track was to be resurfaced (i.e., the ballast replaced) every three years, and the ties and rails replaced every seven. However, since a 2004 resurfacing, neither action had been taken. Metro-North's chief engineer admitted the railroad was behind. He could not explain why, but he said Metro-North was "working towards getting back into phase."
More recently, the NTSB noted, following a derailment of a passenger train in Connecticut two months earlier, which had also resulted from a defective and inadequately maintained rail, the railroad had hired Transportation Technology Center, Inc. (TTCI), to evaluate its track maintenance program. Among the tasks undertaken by TTCI was to survey the entire Metro-North system for areas exhibiting surfacing issues like fouled ballast and poor drainage. One of the 654 such areas it found was the POD.
July 2013 Spuyten Duyvil derailment