​We determined that the probable cause of the derailment was NS’s inadequate procedures to ensure proper repair, testing, and coordination between track and signal departments, which prevented verification of the mechanical integrity of the West Simpson spring switch following repairs and led to a gapped switch point, which allowed a wheelset on train NS-167 to enter the switch point, lose wheel–rail contact, and derail into standing train NS-33K.

​This accident demonstrates the importance of cross-departmental communication and processes to reinforce that communication. In the investigation, the NTSB identified an organizational gap at NS by which track maintenance and inspection actions affecting switch alignment were performed without coordination with the signal department and without testing the switch after the work was completed. NS revised their engineering standard procedures to add a procedure that would alert the signal department to planned work on certain types of switches and require post-work testing. These actions would prevent switches returning to service without confirmation of their mechanical and signal integrity, and could be applicable throughout the railroad industry.​

Employees also must have appropriate training and access to the correct tools to conduct required tests. When the switch at the west end of the Simpson siding showed an out-of-correspondence condition on the day of the derailment, the responding signal maintainer was not equipped to perform mechanical performance testing and therefore could only restore electrical correspondence. After the accident, NS conducted audits across its system, developed training related to spring switch inspection, and equipped signal maintainers with the required tools.