It exists many solutions to lay a garden railway track, most recommended being to lay on a concrete support for a ground based network.
For elevated tracks you can find miscellaneous systems based upon wood boards or, more originally, on high density polyethylen terrace boards which helps a lot the curve laying. See on Family Garden Train.
I foresee to install an elevated track which will at the same time be used as planter box and set with a willow on feet for climber plants. An elevated pot tend to be too dry and it is required to install a buried or drip watering system.
It is possible to find class IV treated boards (which could be permanently in contact with water).
The supporting part shall be stable and protected against wetness. That could be done with shingles which need specific glue or can be welded with a hot air gun.
Another solution might be to install stackable concrete planters.