The immediate cause of the derailment was that the track was unable to maintain the correct gauge as train 1Z75 passed over.

The causal factors which led to the incident were: a discrepancy in recording the poor condition of the sleepers which led to the actual fault being subject to a lower level of surveillance by the Track Section Manager; the condition of the sleepers at Windsor and Eton Riverside station had not been reported on a weekly basis; that action was not taken by the Track Section Manager to replace the decaying sleepers; the Track Maintenance Engineer not following up his request for the work to be undertaken; non-identification of the widening gauge in platform 1 at Windsor and Eton Riverside; and the non-application of tie-bars as a temporary measure.

The RAIB has made three recommendations to Network Rail regarding revisions to its current competency training programme for all staff involved in track inspection; development of a proposal for the periodic measurement of dynamic gauge at potentially vulnerable locations not covered by a track recording vehicle; and ensuring that its procedures for planning audits are amended to a review of the findings from previous relevant audits and action taken.