Dave, I purchased the music wire (that I used to form the railings) and the metal tubing from a hobby shop in my area. They cater to all sorts of hobbies, with a focus on RC cars and airplanes.
I know the music wire is .032 diameter, which is an exact match to the diameter of the MTH railings. I’m away for the weekend, so I’m going from memory, but I believe the inside diameter of the tubing was very close to .032. I purchased multiple diameters of tubing and music wire so I have them available for future projects where I have to match the OEM railings.