A train that derailed near Wallan, killing its driver and pilot, entered a section of track at more than 100km/h, despite a limit of 15km/h.
The speed limit for the passing loop had been imposed after the diversion of rail traffic from a main line.
The speed has been confirmed in a preliminary report on the February 20 crash, released by the Australian Transport Safety Bureau on Friday.
"The train was not able to negotiate the turnout to the loop track at this speed and derailed. All vehicles derailed excepting the rear power car," the report reads.
The derailment of the XPT train from Sydney to Melbourne killed experienced driver John Kennedy, 54, and 49-year-old train pilot Sam Meintanis.
The pilot had joined Mr Kennedy at the front of the train only a few stops before Wallan to help him navigate a 24km-section of track where signalling equipment was damaged, as part of an "alternative safeworking system".
The speed limit for the section from Kilmore East to Wallan was 130km/hour and the train increased towards this speed after setting off.
Earlier that afternoon, the points - or movable rails - at either end of the Wallan Loop had been changed so that rail traffic was diverted from the main line into a loop track.
There was a train notice flagging the change and specifying a speed limit of 15km/h for entering the loop and 35km/h for exiting.
Data from the train show an emergency brake was applied shortly before the train reached the points.
"This slowed the train a small amount before it entered the turnout travelling at a speed in excess of 100 km/h," the report reads.
The derailment that followed left the driver and pilot with fatal injuries, while three passengers was also seriously hurt.
Another 36 passengers received minor injuries, along with five train crew.