I have never seen any recommendations for the amount to tip the Redcaps. My tips have ranged of $5.00 (NYP from the Club Acela to the Adirondack with one medium sized suitcase) to $20.00 (NYP from the Club Acela to the Cardinal with one medium sized suitcase and two roll aboards, and CHI for quickly transferring the same suitcase from a late Zephyr to a waiting City of New Orleans). Incidentally, have always found Redcap service to be some of the best and most consistent in the Amtrak system.