This is my first time posting so it's nice to meet everyone! I'm not trained in physics, but lately I've been very interested in and reading a lot about both Relativity and Quantum Mechanics. With regard to relativity, I found the topic of relativity of simultaneity very interesting. The example often given (which I'm sure everyone here knows) of a train passing an observer on a platform, and light from a source inside reaching the train car's ends at different times from the two reference frames, was very helpful in my understanding. So to help myself better understand relativity of simultaneity, I drew out a couple diagrams of the train example. In general, everything made sense, except for one situation that I'm wondering if someone here could tell me the answer to. I'm sure to someone with a firm grasp of special relativity, the answer to this will be obvious. I'll describe the situation first (which is a variation of the train passing a stationary observer example), and then ask the question. Imagine, just as in the aforementioned example, you are standing in a field with train tracks in the distance. Over these train tracks passes a locomotive that moves very fast (at whatever speed is used in the other examples so this phenomenon is apparent), and pulls behind it a glass train car you can see inside. In the exact middle of the train car is a light source that sends a beam of light towards the front of the train car, and a beam of light towards the back of the train car, at the exact same time (to a passenger on the train of course). Again, exactly like in the train example given by Einstein and others. Here's what I was thinking that I don't quite know the answer to. Let's say one places on the front and back walls of the train photon detectors. These photon detectors are connected to an apparatus on top of the train that consists of the following: timers that measure in nanoseconds, a jack-in-the-box, and a digital billboard. The way the setup would work is that the timers would start the moment the light from the source on the train was emitted (so always at the same time, since it's the same source), and stop when their respective photon detector detected the light. There is a timer for each photon sensor, so the beams' travel times are measured independently. Additionally, as the timers runs, their elapsed times are displayed independently on the billboard (and you, as the outside observer, can see this). The jack-in-the-box does one thing: if the final times from the two timers are the same, it will pop out of its box. If not, it stays inside. Obviously this whole apparatus is part of the train's reference frame. So here's my question. In the usual examples, it is said that to an observer on the train, the light would reach the walls, and thus the photon sensors at the same time. This would be reflected in the timers (they would both display, say, 8 nanoseconds), and the jack-in-the-box would pop out. How would you, the outside observer, see things? It is always said that the observer will see that the light hits the back wall of the train car first. How would the position of the two light beams relate to the number of nanoseconds on the timers that the observer can see? They would obviously appear to be running slower, but would the timer timing the front seem to run slower than the one timing the back (they're both in the same reference frame though, so this I don't get)? And would the jack-in-the-box pop out when the back photon hit for the outside observer, or would it wait for the front photon to hit? I assume that the times would still show up on the billboard the same and the jack-in-the-box would pop out, but I can't grasp how the whole thing would appear to the outside observer. I apologize in advance if there is something super basic that I have overlooked or if this question has been answered multiple times in different texts. Thanks much in advance!