relativity and railroads

[JULIUS RAPER]

	This is a question for all the polymaths out there on the  P-list:
Is there a place where Einstein uses two trains traveling on parallel
tracks as a metaphor explaining relativity--as T. Wolfe later does in OF
TIME AND THE RIVER?  I have a vague memory that either he or Proust
preceded Wolfe in the analogy.
	I need the info for a short essay on Barth.  Your help will be
appreciated.             Thanks, JRR