This time it was the tractor that averted malthusian disaster .
这次是拖拉机改变了马尔萨斯灾难。
The monster 18-wheel trucks that hurtle along america 's highways carry most of the nation 's freight . There are reckoned to be some 1.3m of these " semi-trailers " , as the combination of a tractor unit and trailer are known .