This paper proposes a decisive laboratory experiment capable of empirically distinguishing between absolute and relativistic notions of simultaneity. The experiment employs two perfectly synchronized lasers mounted on a moving train and two independent receivers – one on the train and one on a stationary platform. At the precise moment the train’s midpoint passes the platform’s midpoint, both lasers fire simultaneously. According to Einstein’s relativity, both receivers should record simultaneity. However, if simultaneity is absolute and light speed is not universal but additive to the source’s motion, the platform receiver will detect a measurable time difference. This experiment’s clean design avoids synchronization ambiguities and provides a direct test of the relativity postulate itself. Keywords: Simultaneity, Special Relativity, Absolute Time, Light Speed Constancy, Train Thought Experiment, Lorentz Transformations