One problem with RR ties is the Creasote. I have tried several types of paint and have yet found anything to seal it up. My latest was trying to paint over some 2x6's that were creasoted 40 years ago. For PT lumber here is another idea, You could use 15lb felt paper, --Normally made for roofing underlayment- between your pt lumber and soil. About the time the felt paper rots out, the leachable amount of Arsenic should be gone. Byron