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You would think trains do less damage to the environment than buses, right? All those nasty bus exhaust emissions and all. But how do they actually stack up against each other once you do the numbers and bring in the scientists?
Believe it or not, buses and trains use the same amount of energy throughout their life. And in some circumstances, cars are also in the same ballpark!
According to transport researcher Jeff Kenworthy at Perth's Murdoch University, on average Australian trains burn only 0.53 megajoules of energy for each passenger-kilometre (MJ/pkm) while buses use 1.66 MJ/pkm. That's three times more energy to carry one passenger one kilometre.
These are, of course, average figures: the amount of energy used by buses or trains depends on the type of fuel used and number of passengers carried. In Australia the majority of buses run on diesel, although increasingly CNG (compressed natural gas) and LPG (liquefied petroleum gas) buses are being used.
Trains can be diesel or electric.
Kenworthy's figures only include energy required to operate the buses or trains. For example, in the case of electric trains, he didn't include transmission losses or the different power station efficiencies. The question is, is this energy significant?
A more comprehensive answer would be one based on a cradle-to-grave (life cycle) assessment, one that considers the energy used to the extract the raw materials, manufacture the train or bus, operate them and then dispose of them.
Manfred Lenzen of the University of Sydney uses a method called ‘input output analysis', which enables the assessment of environmental impacts up and down the supply chain.
As well as operating energy due to the combustion of fuels, his analysis includes energy to extract, refine, store and distribute fuel such as coal, oil and gas and electricity generation. He also looks at the energy required to mine and manufacture materials to make the buses and trains, and to construct roads and railway tracks.
After studying all public transport systems in Australia, he came up with a startling result: indirect energy use is a significant part of total energy use: in some cases, up to 65 per cent of the total.
So looking at just operational energy is really only a small part of the picture.
Lenzen found that, cradle-to-grave, buses and trains use almost the same amount of energy per passenger-kilometre.
Urban bus and urban heavy rail both use 2.8 MJ/pkm while light rail uses slightly less at 2.1 MJ/pkm. Cars, incidentally, use 4.4 MJ/pkm, which is more than twice that of public transport.
Peak and off-peak
Lenzen points out that results vary considerably depending on the occupancy levels. Since loading levels change from nearly 100 per cent in peak periods to 10 per cent in the off-peak, the off-peak energy use of trains and buses increases threefold.
Cars, on the other hand, become more efficient in off-peak periods. There are, on average, more people in the car and the roads are less congested, compared to the lone drivers in peak hour traffic jams. It turns out, in the off-peak period, cars with two or more people have similar energy intensities to off-peak buses and trains: between 3.5 and 4 MJ/pkm.
Greenhouse gas emissions
With the focus currently on climate change, how do buses and trains compare in the emission stakes?
Lenzen calculated greenhouse gas intensity of bus and trains - that is, the total kilograms of carbon dioxide emitted per passenger-kilometre. He found that light rail and buses were similar at 0.2 and 0.22 kg carbon dioxide (CO2) generated per pkm respectively, while heavy trains were more than double at 0.7 kg CO2 per pkm.
Heavy rail had higher emissions due to a higher resource input required for the construction of railway lines.
Buses and trains consume almost the same amount of energy throughout their lifetimes. But heavy rail produces slightly more greenhouse gas emissions.
Compared to cars, buses and trains on average have a much lower environmental impact, although most of this benefit occurs in peak hour.
So next time you're considering how to get to work - it's confirmed: public transport is the way to go.