The first step is making a sketch of the desired piping.  The drawing below is not to scale, it simply shows the routing of the new piping.  Existing pipes are not shown. I will not measure the wire exactly.  Instead, I will lay it in and bend where appropriate.

Materials:

• Bell and whistle cable: Fine copper wire pulled from stranded wire
• air, sand,and handrails: 0.050” brass wire
• Junction box: 0.100” inch thick plastic from the scrap box, 0.200 square. Perhaps a plastic tie clipped and filed.
• Steam supply to air pump: 14 gauge, 0.064"

Wire hangers:  0.050” inch brass sheet, cut into strips 0.100” wide

Most pipe will be 0.050 brass wire.  The sister engine, a brass import, uses 0.050 wire for all the piping, including air, sand, and handrails.  I could find the exact prototype pipe diameters and use wire of those sizes, but the two engines are supposed to be in the same series and will be seen together.  They will look similar if I continue to use matching sizes.