I'm not sure the question belongs to classical physics, I apologize in case it's a no. Imagine a train or any object following a straight line motion. Suppose there's no friction between the ground and the train, such that its motion would go on forever. Now suppose that the train emits photons from its backside, parallel to the ground. So its "losing" energy and should go slower I believe. However I remember my professor said that even though photons are massless particles at rest, they carry a momentum. Hence... I'm tempted to say that the train should in fact go faster instead of going slower. I'm sure I'm confusing a lot of things. Can you explain me what would really happen in such an idealized situation?