This is Sleepers you can use outdoor with aluminium or wood rails (still testing with wood).
I have noticed that the rails on my old designs are sagging a little if it is to hot in the sun or if you are using live steam loco. The aluminium flat bar i am using in the pictures are 3 x 15 mm.
My next big release will be a bigger loco than my old one, but because my old 3d design software stopped working i lost around 60% of my work. and i am forced to redesigning a lot in my new software, im now using Shapr3d . if you have a ipad pro and a pen i can highly recommend this software