IRISH RAIL this week blamed last October's collapse of the Cahir viaduct in County Tipperary on the interaction between simple axle goods trucks and the ductile bridge structure.
The track operator said the 1852-built bridge collapsed due to 'a particular unique circumstance. . . caused by a dynamic combination of a laden wagon and its interaction with the track features of the bridge.'
It said the failure of the bridge last October could not have happened under loadings from a passenger train fitted with bogies.
The bridge collapsed as a freight train passed over it (NCE 23 October 2003).
Investigators found that a simple axle cement wagon derailed causing the derailment of other trucks and triggering the collapse.
Five wagons fell into the River Suir and seven onto the river bank.
The three-span bridge comprised two outer wrought iron plated box girders carried on masonry abutments and connected with a series of iron cross members.
The unballasted rails were carried on a pair of timber way beams. The derailed wagons progressively destroyed the rail supports, the bridge deck and the main cross beams.
Initial examination could not identify a track or wagon fault, but laboratory and site testing found the rigid ride of simple axle wagons passing over the variable stiffness waybeam bridge 'could create a situation where is was possible for a derailment to occur.'
The report discounts the load of the wagons, a train defect, vandalism, or the operation of the train as factors in the accident.
Since the accident, Irish Rail has reduced speed limits for trains with simple axles from 64km/h to 48km/h.
It claims modelling shows this slower speed is enough to prevent derailments on similar bridges.