Maybe we could use a couple of more real life thought experiments.
You are on the top of a mountain and you setup your Muon detector.
and will be able to count the mouns before they decay because,
1) In the frame of reference for the Muon lengths are contracted.
2) In the frame of reference for the Muon lengths are expanded.
3) Neither of the above.
Or instead of a train we use the LHC and particle beams instead of lasers.
The new collider is turned on and you press the button to fire two particle beams at each other at .9999c. When a few million revolutions have completed you trip the switch for them to collide.
When you look at the screen for the results.
1) You as the observer would see nothing as the beams are in a different frame of reference and for the particles in the beams time is passing slower and they haven't collided yet.
2) You as the observer would see the collision as the beams are in a different frame of reference and for the particles in the beams time is passing slower so they haven't collided yet (Added bonus we just created a time machine).
3) You as the observer would see the collision as time dilation didn't occur.
Edited by Guiri, : No reason given.