Are trains really Green?
According to Roger Kemp of Lancaster University, trains produce as many greenhouse gases on a per-seat basis as cars and planes. Standard high-speed trains in the UK are no better than modern economical cars - and the new superfast trains are as bad as planes over 600km.
Engineering Dept News - Trains Lose Steam in Green Travel Race
There's a fuller discussion here: http://www.aiada.org/article.asp?id=17840
However, suburban trains and metros are much more efficient than cars, mainly due to higher passenger density, but also due to getting from A to B much more quickly than cars that are stopping and starting all the time, using more fuel in the process.