Instantaneous action at a distance may occur due to an experiment being performed within a train. A particle may decay in the middle of the train, and when decaying it may split into 2 entangled particles that fly apart in opposite directions. Let's say that these particles are photons. When measuring properties of one particle when it reaches one end of the train, it is said that this will instantaneously affect the other particle at the opposite end of the train.
However, if this experiment was performed again as the train passed by a train station, observers at the train station do not see the two photons reach the two opposite ends of the train at the same time, thus to them there is no instantaneous action at a distance taking place at all.
Thus how is it that what appears to the obsevers on board the train to be instantaneous action at a distance is physically confined to this one frame of reference only ?
Edit: I will add to the details here, that the photons are said to be in superposition of states until being first measured at one end of the train or the other. Which end measures first may depend on the accuracy of the experiment that was set up. But either way both photons are said to be in superposition of states until that moment that they reach the train ends, +/- measurement error.