Then in order for the length contraction to happen from the beginning, both ends should be in different states of motion relative to a platform observer. And in order for the train to regain its original length after coming to a stop, both ends should end in different states of motion too assuming in all cases that both ends have the same states of motion relative to the train observer or in other words, are space-like relative to the train.If you consider length to be part of "mechanical reality" then length contraction is also part of "mechanical reality" and consequently "mechanical reality" is frame variant.
But even this underestimates the effect of the motion on length because the length contraction should be associated with moving objects no matter when its 2 ends start to move relative to a platform observer, because its physical existence is related to the way the observer watches the light rays bouncing between 2 ends and not related to a mechanical translation of the rear end. And because in the first case, the length contraction is a result of physical constraint imposed by the invariance of speed of light while in the second the mechanical translation becomes a trigger to cause such invariance of c. So length contraction can not be the cause and the result at the same time.
Even the starting of the motion is not clear relative to all observers. For example, if the 2 ends of the platform start moving simultaneously relative to the train observer, the rear end of the train should start moving before the near one relative to the platform observer which makes the length contraction a mechanically wise for him. But if the 2 ends start moving simultaneously relative to the platform observer no length contraction appear, contrarily to many authors argued that there would be still length contraction and the train will go under strain,,, see Bell`s spaceship paradox.
So after those 2 arguments, the length contraction and inductively the length itself at the beginning and at the end of the motion is not only ambiguous but may be physically inconsistent too.