On U.S. Highway 65 recently somewhere near Dumas, Arkansas, goin' south. In case you can't understand the last part of my little diatribe on relativity, I'm saying that the essential thing about relativity is whether you can do an experiment to detect whether you're at rest or you're moving. The answer in almost every situation (see below for exception) is no, you can't. Whether you're in straight line motion, or going around a curve, or accelerating upward in an elevator, or freely falling you are in a rest frame. Remove the visual cues, and the Newtonian frame of mind that says there's an absolute rest frame, and the experiments you do to try to detect your motion will fail. For the speed of light to be constant for everybody, no matter how they are moving relative to each other, the sensory information you have is only interpretable as something or somebody moving at constant speed relative to you, or for your apparent accelerated motion, as you being at rest and acted on by a gravitational field. We should, but so far don't, describe relativity in terms of rest frames moving with respect to each other rather than "reference frames" moving with respect to each other.
The one situation in which you can detect that you are moving is the earth's motion on its axis, which can be detected by setting up a giant pendulum suspended by frictionless pivots so that the pendulum swings in a single unchanging plane of motion as the earth turns (was that the name of a soap opera?) beneath it. However, the earth is so big it's like an unmoving flat surface as far as the relative motion of my car and the train (and the trees and highway attached to the earth) are concerned. Try imagining just me in my car and the train in empty space moving relative to each other. Then you can understand how they're both rest frames as far as the people in them are concerned.
