To begin with, there is a difference between energy sources for rail transport. Pollution from
railways varies depending on whether the train is driven by electric or diesel power, the main difference
being in the location of the pollution and in the emission rate. The emission rate is a little lower in thermal
power stations than for on-board engines; it is appreciably lower -- virtually zero -- for nuclear power plants
and hydro stations. This phenomenon may explain the difference between estimates of rail pollution for a
country, and shows the care required when transposing results from one country to another.