V/Line has banned trains from running on several of the state's most important freight rail lines once the temperature reaches 33 degrees, right on the cusp of a major grain harvest.
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The ban was issued without warning and has grain farmers reeling, saying it will effectively shut down their operations on hot days.
It could also force them to put hundreds more trucks onto damaged rural roads.
The new heat-related threshold of 33 degrees, issued late on December 9, is believed to be in response to two freight train derailments near Ouyen in far north-west Victoria in December last year. The Australian Transport Safety Bureau has almost completed its investigations into the derailments.
Heat-related speed restrictions are enforced because tracks expand in hot weather, increasing the risk of a derailment on poorly maintained railways.
Last summer the restrictions were triggered at 36 degrees.
Lines covered by the new ban stretch across the grain-growing heartlands of the Mallee and Wimmera regions and include critical links between Donald and Mildura, Echuca and Toolamba, Shepparton and Tocumwal and Dunolly and Manangatang.
They have been applied between midday and 8pm or 10pm.
Rail Freight Alliance spokesman Reid Mather said the lower temperature threshold would hurt grain exporters and appeared to have been poorly thought through.
"I don't know why they'd be making this call at the height of the grain harvesting season in Victoria," Mr Mather said.
"I don't know how they are going to get product to port. If you close it for eight hours they lose their windows so effectively you've stopped the system."
Grain growers would be forced to turn to trucking companies to get their harvest to market, which would have the twin consequences of reducing road safety and putting increased stress on damaged rural roads, he said.
The rural road damage caused by greater use of trucks to move grain, as well as the safety risk to other road users, was highlighted in a Victorian government report into grain trains released in 2011.
Quambatook grain grower Brett Hosking, who is also president of the grains groups of the Victorian Farmers Federation, said the new restriction "emphasises the need to invest in rail".
"If we had a state of the art rail system, we wouldn't be looking at these sort of restrictions," he said.
Mr Hosking said it was not uncommon to have a 33-degree day in northern Victoria and warned that the heat restrictions would push more trucks on to country roads, during what is expected to be a record-setting grain harvest.
"Rail is the most efficient means of moving bulk produce, we also know it's the safest means of moving bulk produce. And to see the state go backwards and move that task from rail to road - I think for most people it seems like a step backwards, simply on the back of what is just an average summer's day," he said.
Mr Hosking's farm is on a line where the 36 degree threshold has been maintained.
Asked how often the temperature reached 36 degrees in Quambatook, he said: "Probably more often than it drops below. Thirty-six degrees is probably what we'd expect for January/February weather."
V/Line and the Andrews government have been contacted for comment.