I'm currently on a train on my way into work. It's not how I normally get in but, having taken one into town the other day, i've now got a cars on the opposite side of london from me.

The figure of 60g of CO2 per passenger kilometre is often accepted as accurate for a commuter train. But if something that weighs 550 tonnes - 10 carriages at 50000kg (written on the side) each carrying 500kg of passengers (70 people at 70kg a head) - keeps accelerating and braking as much as this, it's difficult to see where this much-heralded environmental friendliness is coming from.

Notice something else about the train weight? It takes around 700kg of train to transport each passenger - for a mass transit system, that's horrendous.

And the route is longer. God, I feel guilty taking this train.