Cambridge 2000 memos
People who use trains all over the world, and in particular in the UK, have their tickets subsidised by the taxpayer. This is generally considered a "good thing" because trains are supposed to be an environmentally friendly way to travel. However this widely held belief misses more subtle points.
If you need to travel from A to B then it is (probably) the case that going by train rather than by car is more environmentally friendly. This is not always true because it depends where A and B are relative to the actual train service (i.e. you need to get to the train before you can use it), and whether the train you are using happens to be full or empty. But perhaps as a general rule on average trains are (say) two to five times more environmentally friendly, or more honestly, less environmentally hostile, than cars.
The problem is that life is not so simple as the above implies. Cambridge house prices are less than London house prices, so it is convenient for some people who work in London to live in Cambridge and take the train to work. And it is also often the case that for a couple who live in Cambridge then one of them will work locally (e.g. at the university or in a hi-tech firm) and one of them will commute to London.
People who commute to London to work are generally much more highly paid than people who work in Cambridge. Thus they can afford to pay more for houses, which means people who work in Cambridge are often forced to live outside of Cambridge in one of the surrounding villages and suffer the hassle of driving into Cambridge every day to get to work (which local government does nothing to improve and usually only does things which make it worse).
People who have to drive five to ten miles to get to work in Cambridge (often including a stretch along the "highway from hell", the A14) are treated like environmental vandals by government and have to pay over the odds for the privilege, whereas people who take the train fifty miles from Cambridge to London are treated like environmental saints and are supposed to be given tax subsidies for doing so. But in fact the environmental damage caused by the London commuters is probably greater, since the distance travelled is so much further (especially when you add the extra travel at both ends).
People who work in Cambridge (teachers, nurses, etc.) who are relatively poorly paid are expected to subsidise the journeys of London commuters (IT staff, bankers, etc.) who are relatively well paid. Is this fair or sensible? (Well, people who earn more pay more tax, so in fact most subsidies go the other way.)
The general environmental point is that it is not just the means of travel to work that matters but also how close you are to your workplace, and it is far better for people to work locally. Environmentalists and politicians like to decry road building, because it only encourages people to live further from their workplace. But in fact subsidising commuters travelling to London on trains has exactly the same effect. You could argue that getting to London (by road or rail) should be made as difficult and expensive as possible so that people who work in London live in London.
Cambridge 2000 memos