Good luck on that one....I always had trouble finding a quiet spot. The problem with the one the PP posted is if it is the one I am thinking of, the train will be going by there about every 10 minutes, most likely blowing their whistle!!
The one time she took a nap in the stroller in MK, I sat at the entrance way btw Fantasyland/MTTF and Tomorrowland. It seemed pretty quiet - no trains, minimal music. It's hard, if they are sound sleepers it will be easier, but my DD is a light sleeper during nap time. Every spot I thought about, there was either a train, loud music, a show or a ride where it wasn't quiet! We usually ended up going back to the room - it was easier...