This has been bugging me for a while and it may seem like a rather stupid question. I've been reading "Relativity" by Einstein and in section 7 of special relativity (the apparent incompatibility of the law of propagation of light with the principle of relativity) Einstein describes a thought experiment in which a ray of light is sent along a train track and a train is travelling in the same direction as the ray but obviously at a much slower velocity. He then goes on to say that we would expect the velocity of the ray of light relative to the train to come out to w = c - v. I understand it up to this point, but what I don't understand is why this is a violation of the principle or relativity. As stupid as this may sound I would appreciate any help. I get the feeling that the answer is simple but for some reason I'm just missing it.