If the point of the air track is to eliminate friction from consideration, let's make the situation even simpler:
Imagine a completely empty universe. Total blackness, no objects, no light, nothing.
Now add two clay lumps, and an invisible observer (you).
One clay lump appears (to you) to be motionless. Another clay lump appears (to you) to be moving towards the first clay lump at high speeds (feel free to make this speed as close to the speed of light as you like).
Now ask yourself these three questions:
If you (the observer) were to zoom over to the 2nd clay lump as it approaches and match its speed and direction so you were gliding next to it, how would the 2nd clay lump appear to be moving to you?
How would the 1st clay lump then appear to be moving to you?
If you were suddenly put into this situation, how would you tell if you were floating next to clay lump 1 or 2 (assuming they look identical)?
That should give you your answer.