To begin I will explain briefly a classic thought experiment used to demonstrate simultaneity;
There is a 100m long train rushing towards a 100m tunnel at a fraction of the speed of light, from an observers perspective the train is shorter than the tunnel and so the observer can safely trigger guillotines at either end of the tunnel to briefly fire and not hit the train when the train is inside, however from the trains perspective the guillotine in front fires first and the one behind afterwards as to not hit the train.
However I have thought of a slight modification to the experiment that appears to break relativity;
Imagine a sensor on the front of the train which detects when a guillotine passes infront of it, this then sends an electrical signal down the train to a light at the back of the train which, I think, would turn on at the same time as the back guillotine drops, from the trains perspective this is fine however from the observers perspective information has travelled instantly, the firing of the first guillotine has turned on the light with no delay. What is the resolution to this? One possible resolution to this that I have thought of is the the simultaneous events of the light and the back guillotine in the trains perspective is not simultaneous in the observers perspective, however this doesn't seem to make sense as its a simultaneity paradox within a simultaneity paradox and I can't see any reason for the two events to be non simultaneous (they're in the same place moving at the same speed)