Real objects differ in mass and composition, thus inertial frames are inherently
unrealistic. Anyone who doesn't believe in a preferred frame should try this
gendakenexperiment: wearing football pads, run into a "stationary train" at the
"relative velocity" of 9 mph;
after a few months of recovery, try having the conductor start the train and
with you standing absolutely still on the track, run into you at the relative
velocity of 9 mph.
I would much rather prefer to be in the first frame.
REal objects have to have real velocity or else they couldn't properly interact with
other objects as they make contact. The only time velocity is totally relative
is in the situation where the objects are identical in mass, shape and composition
and in a total vacuum- in other words, never.
