My advice is always to work with materials you are 'happy' to work with.
Locally we have railways built with wood and also metal.
Mine is in wood as I enjoy working with wood and it's easy to cut, drill and screw.
Bot if you like metal and pop riveting or other ways of doing it, I would make sure that it is material that you understand and enjoy working with.
Mine is all wood with posts sunk in the ground and 2 x 3 and 2 x 4 cross pieces all screwed together with a plywood top cover in self adhesive roofing.
David Leech, Delta, Canada