A BBC report which explains:http://www.bbc.co.uk/news/world-europe-15429744

Which cites:
"He wasn't motivated by politics but rather social envy," said senior police official Oliver Stepien. "He said in essence: 'I've got debts, my life stinks and others with fancy cars are better off and they deserve this'."

Earlier this month, the Berlin railway system was hit by arson attacks. Explosive devices were placed in a number of locations, including at least one on a high-speed rail track. A previously unknown left-wing group, calling itself the Hekla Reception Committee, said it had planted the devices.