But let's just say that I've ridden on numerous German cities' subways and by far the filthiest are those in Cologne, where there are no garbage cans. (I'm not sure if eating on the train is actually illegal anywhere, or where exactly...such a rule is sure as hell not enforced anywhere in Germany though; and the rule on no alcohol consumption on the train is sometimes enforced but usually not.)
Garbage cans are unlikely to be installed on the Metro for the simple reason that they provide an excellent place to dump bombs. The simple "inconvenience" of not being able to munch on your Twix bar is worth not having another 3/11 or 7/7.
There used to be garbage cans inside Metro, but they were removed after 9/11.