An investigation into the train derailment near Knaresborough on November 7 found that the employee responsible for signalling was off work sick.
The 6.52am train from York to Harrogate, carrying four passengers at the time, derailed near the Knaresborough rail station.
A report from the Rail Accident Investigation Branch (RAIB) found that the train was due to remain on its main line and not to use the crossover.
The report explained that the train's movements were controlled from the Knaresborough signal box but the resident signaller had not operated the box for a 'number of months' due to sickness.
Another signaller was appointed to work the signal box the afternoon before the derailment, causing disruption to the line's rail service.
The RAIB confirmed they will now investigate the process in place for appointing relief signallers as well as any relevant underlying management issues.
None of the train crew or passengers were injured as the RAIB continue to investigate the sequence of events that caused the train to derail.
They will also consider the actions of the signaller and train driver and the design of the signal box.