Why do we always need to define simultaneity in relativity. The moving train with a light signal at the mid point reaches the engine and caboose at the same time and gives us a way to define simultaneity. But what is it's purpose. if G = gamma = 1/ ( [ 1-V*V/C*C ] ) ^1/2 And from the Lorenz equation T' = ( T + [V/C*C] *X ) *G It appears that T' The time seen when a fixed observer is looking at a moving train is a function of X. As the caboose passes the observer the engine (with a bigger X) is older than the caboose. What is the purpose of simultaneity?