The design speed of the turnout is 40 km/h. The usual rule of thumb is that at roughly twice the design speed the inner wheels will unload on the points, leading to a derailment on the following reverse curve. This was well above twice the design speed. I guess the N and the N set have a low centre of gravity and stiff springs. Lucky.
The signalling was working as designed, with the Home signal showing low speed caution, and the auto in the rear showing normal speed warning (i.e. next signal is at stop). This was the original NE SG CTC arrangement. Moves into the loop were signalled using a low speed aspect as there was essentially no overlap at beyond the departure home signals.
The driver had never been signalled into the loop at Wallan before, and, in interview, said that he expected a medium speed aspect into the loop. The report notes that the driver didn't respond at all to the automatic at yellow.
The overspeed variant of TPWS would have prevented this incident, and is installed on the V/Line RFR lines for this reason. It is not installed, however, on the ARTC lines.
The ATSB report focuses on the reporting of the incident. The buffet attendant reported the overspeed, because they had injured passengers.