My understanding of the POE is that if you restrict your observations
to a limited region of space THEN you cannot tell the difference between
a UNIFORM acceleration and a constant gravitational field.
IMHO it doesn't imply that a rational view of ANY acceleration is
the existance of a comparable gravitational field. If you take the view
that Newton's laws can be made to apply to an accelerated frame of
reference by introducing special forces, that is just D'Alembert's
principle ( Fspecial = - M Aref , where Aref is the acceleration of the
reference frame).
The original question was how to respond to the student ('Charles')
who asked where the kinetic energy of a tree went when viewed from a
stopping train. I don't think either the POE or D'Alembert's principle
are appropriate responses at the Freshman level.
I still think that the student who really wants to "know where the
energy went" and insists on the view from the train should be asked
what the motion of the tree is as seen from the train as it is stopping.
I would describe it "as if a force were acting on the tree", and wind
up the discussion be reiterating the problem of using an accelerated
reference system.