The work was completed by Professor Dan Jaffe of the University of Washington in Bothell and several students, and relates the results of an extended observational study above the tracks in north Seattle and in the Columbia Gorge.

The research was considered too controversial for state agencies and so the University resorted to crowd-funding in order to complete the task.

Mass released the findings to his website, saying the diesel effluent degraded the air quality in neighbourhoods around the tracks.

“There is clearly coal dust coming off the trains, even when it is not visually apparent [such as north Seattle],” he said.

“The trains cause substantial increases in traffic jams, worsening pollution from cars, and impacting public safety and the local economy.

“The planned coal on these trains would substantially contribute to CO2 increases in the atmosphere.

“A significant portion of the pollution from the burning of coal in Asia will make its way back to the Northwest, further degrading our air quality.”

Mass argued the jobs created in trains and terminals did not outweigh the negative environmental consequences, and called for stricter regulations and eventual closure of the coal transportation lines.

However, the conclusion Mass drew has been criticised as being presumptuous and inferential, especially when he concluded that the US not selling coal to China would reduce the amount of coal burnt in Chinese power stations.

Critics on Mass’ website said the conclusions were characteristic of trains in general, rather than specifically coal trains. They argued diesel effluents were problems associated with any locomotive.