In these carbon-aware days, train companies are starting to push their credentials as the low-emission option, eager to exploit the negative press the aviation industry attracts.

Last April, Eurostar announced a target of reducing CO₂ emissions by 25% per traveller journey by 2012 and at the same time claimed that the CO₂ impact of a Eurostar rail journey between London and Paris or Brussels is currently at least 10 times less than the same journey by plane. The UK’s Virgin Trains has just launched its “Go Cheaper, Go Greener” marketing campaign, claiming that a journey from London to Liverpool on one of its Pendolino trains emits 76% less CO₂ than a domestic flight or car.

But are trains really as green as operators claim? To answer this question is to enter a complex world of data, carbon calculators and scientific uncertainty, but slowly methodologies are emerging that allow us to more accurately measure the carbon emissions of planes, trains and cars.

According to the UK’s Department for Environment, Food and Rural Affairs’ (Defra) recently updated figures, trains are the lowest-emitting form of transport, unless there are four people sharing an average-sized car (a rare event unless a family holiday is involved).

An average-sized petrol car emits 209.5 g CO₂ per passenger km, while an average-sized diesel emits 198.7 g CO₂ per passenger km. If that seems much higher than the figures quoted by carmakers, it’s because Defra adds a 15% loading factor to take account of real-world driving characteristics and the use of air conditioning. For a domestic flight from London to Scotland in a Boeing 737, with a load factor of 65%, the figure is 158 g CO₂ per passenger km. For short-haul flights, 65% full, the figure drops to 130.4 g CO₂ per passenger km, while a long-haul flight in a 747 with a load factor of 80% emits 105.6 g CO₂ per passenger km. Diesel and electric trains on the national rail system emit about 60.2 g CO₂ per passenger km, rising slightly to 65 g CO₂ per passenger km for light rail. Using UK offsetting company Climate Care’s emissions calculator, which uses a different emissions calculation method for air travel — based on a specific journey rather a set journey/km — comes up with broadly similar results.

The truth is that it’s impossible to get exact figures on comparative emissions at the moment because they simply don’t exist: there are too many variables involved, and the science (as yet) can’t cope. For instance, as soon as trains enter France their emissions drop substantially because most run on electricity provided by low-carbon nuclear plants. High-performance diesel trains of the type that dominate the UK are comparatively less environmentally friendly. Scientists are not really sure of the effect of CO₂ and other greenhouse emissions from aircraft at altitude.

“Reliable data for public transport – such as rail, coach and plane – is a work in progress, and the increased interest in carbon footprints and offsetting is driving improvements,” says a spokesperson from Climate Care. “We need to push for a situation where the actual carbon impact – measured as accurately as possible – is built into the economy, so we can make sensible decisions based on fact rather than emotionally charged assumptions. But to avoid the triumph of numeracy over common sense, it is better to start measuring and acting now than to wait for perfect numbers.” EB