Well you are taking the very first baby steps in understanding that you have been teaching nonsense all your teaching life.
I set up no principal, and abandoned nothing.
Your problem is still in thinking that by some magical means, "attaching" an imaginary construct has some bearing on reality.
This is delusional thinking, some form of mental illness, surely.
Galileo and Newton were discussing the very simple, but maybe overlooked fact that motion of objects appear to be moving differently when compared to different observational positions.
But nothing they said could be considered magical.
A ball moving at 10 kph on a moving truck with strike a pedestrian with a force of the combined speed of both ball and the truck.
This is not rocket science, and its only use is in the calculation of the effects of the motion of objects, on associated objects.
Einstein however twisted this simple understanding of very basic Physics, by adding actual magic, mysticism and an almost religious faith aspect when he claimed that by virtue of only relative motion, Time, spacial distance and Mass must be actually altered, but only according to special individuals, not for all. This is not Physics, its the talk of an irrational mind.
The sentence you wrote that was important is the recolonization that "The Mechanical Physics of this part work the same no matter whether you are putting it on the Earth frame of reference or the moving train frame of reference.". Therefore, you surely must recognize that it's dimensions, Mass and Time can never alter, irrespective of which 'frame' you care to invent.
Its you who is setting up a principal, and then soon you will want to abandon it in order to embrace your magical, mystical religious faith.
Because of Galileo's and Newtons simple laws of Physics, (and any rational persons Physics), Time, Mass and Lengths can never alter one scrap just because some deluded person decides to observe the action from some alternative location.
So please don't put the frame of reference on a moving train, or another planet, or on a merry-go round, because my machine might stop functioning, due to its mass and size changing, and the machines delicate timing will surely get messed up, and all by MAGIC!
No my friend, in fact you CANT put the frame of reference ANYWHERE, because its does not exist! It's imaginary, and plays NO PART in the actual physical processes.
My machine will work exactly the same, irrespective of where you are, what speed or direction you are moving in, and what tricks of perspective are affecting your clear view of the machines operation. What you subjectively BELIEVE you are witnessing, has NO bearing on my machine in any way, shape or form.
Its only in your head!
This is not the study of Physics, its the study of the state of your mental processes.