The bridge at Barrow on Soar partially collapsed onto the Midland Main Line below on August 1 causing rail chaos for commuters as services were suspended.
The RAIB said: “At the time of the collapse, drilling work was being undertaken to investigate a depression in the footpath on the south side of the bridge.
“There were a number of pre-existing defects in the bridge structure which Network Rail had been monitoring for some years.
“Shortly after drilling commenced to take soil samples for analysis, cracks appeared in the footpath, and then the adjacent wall, footpath and the drilling rig fell onto the railway.
“Workers engaged in the drilling work were able to get clear and no-one was injured.”
Rail services through the bridge were suspended until two days later.
The RAIB said: “Our investigation will examine the sequence of events leading up to partial collapse of the bridge, the construction and maintenance of the bridge, its recent history, arrangements for protection of the railway during the works and any relevant management issues.”