Hi. I am learning special relativity and have decided to take Einstein's train example to practice some calculations. I have given the train speed v=1/4c and have decided on distances that make the side observer's space and time calculations straightforward. I have calculated everything using the invariance of the spacetime interval equation. (ST^2 = T^2 - S^2) for timelike intervals and (ST^2 = S^2 - T^2) for spacelike intervals; where S is the spatial distance, T the temporal distance and ST the spacetime interval between events. However, whenever I try to think about how I would approach the matter of the train rider's interpretation of the time and space coordinates of the side observer's seeing both lightning flashes, I draw a blank. Can you think of the easiest way to approach this using just the spacetime interval equation? edit: Also please note this isn't a homework question - I wanted to do this for myself. This is why I placed this thread here rather than on the homework forum. Thanks!