I can't tell you how HE built it but the book "Scenery for Model Railroads" by Bill McClanahan has a section on donig it.
Much of the masonry in John's later work was done by carving plaster (e.g. the photo on the front cover). That bridge was done quite early so he might have used a different method.
I would do it by making a core from hardbord or thin plywood (with, say, cardboard on the inside of the arches) then coat in plaster or Polyfiller and carve when dry but not totally solid. I have heard that hydrocal is a lot tougher to carve.
If you look at the photo on page 50 it looks like the arch at hte right is made of cardboard that has had card "stones" glued on but i don't think it is as nice as the other end.
Actually looking at more photos i think you caould make it from contoured plastic or card (you can get sheets of plastic formed like faced stone like that). You would have to carve some of the stones (keystones etc) from plastic but the bulk of the tedious work would be done for you.
Just my thoughts.
Neil