I played around with 'handlaying' tubular track from something I saw on another forum, it isn't that hard.
1)take the metal ties off the tubular rail
2)have pins on both ends of the rail, and connect each end to a piece of tubular (intact) track. At this point, the rails will be lined up to the right gauge.
3)I bought some 1/4" wood (someone else said 1/4x1/2) at a craft store. I cut them to a length that was roughly what a real tie would be.
4)Since I was just doing a proof of concept, I 'stained' the ties using a dark brown/black market I thought was close to what real ones color was.
5)I glued to the ties to the rails (the ones I took the metal ties off off) using Gorilla glue, putting them on spaced roughly what a prototype rail would be.
6)Once the glue dried, I then also 'weathered' the rails using a brown rust color marker.
I don't have a picture handy, but it didn't look half bad. The person who I saw do this actually spiked the rail to the ties, which was a bit to detailed since I was just playing around.
Since you are not powering the rails, being outside might not be that much of a problem, though you still may want to paint the rails , since the tubular track metal finish when it rusts doesn't look like rusty real rails (sounds weird, but that is my thought.
You could obviously keep the metal ties and put wood in between, but I didn't like that look.
The biggest cost for this likely will be the wood for the ties, using cedar or pressure treated wood makes sense, but I don't know how much that would cost you (prob cheaper if you could rip the ties from a piece of wood, versus buying something at size), the wood may be the most expensive cost of doing this especially if you use weather resistant wood like cedar or pressure treated. Might be cheaper buying used gargraves, you should be able to price out the alternatives.