According to the May 2006 Discover Magazine (pg 18), Svensk Biogas is running a commuter train not just on the methane from cow manure, but is now adding the live cows themselves to the mix.

It takes 30 cows to move the train 75 miles, for an average 2.5 mpc.

I hardly believed this development at first, so checked google, and sure enough-
http://news.bbc.co.uk/1/hi/sci/tech/4373440.stm

Being green doesn't necessarily mean tree-hugging, but this coldly logical biomass approach really bothers me. Like turning cows into cannibals (and getting mad cow as a result), it brings the word abomination to mind.
Also the phrase "Have we no decency?'

In related developments, San Francisco is collecting pet droppings, and experimenting with methane power.

I'll make another 'modest proposal': the obvious elephant in that room is human dung. Then, of course, if that proves insufficient, there are illegal aliens, detainees, and other people without rights in our society, who could be conveniently disappeared... but you've seen that movie.

At the very least, if we justify powering vehicles with food & societies waste, can Soylent Green be that far in the future?