MY Experience. I have used the thicker 1/2 hardi backer board, painted, as bases for buildings over gravel or on patio blocks and it holds up decent enough. I had some of the 1/4 stuff as scrap so I tried it for a short retaining wall behind the RR in direct contact with dirt on one side and it became brittle and cracked up like in Kevin S’s photo in about 2 years.
I have been using the 1/4 backer board to make roof panels for my various buildings and painting them brown or black. The material holds up well (so far) if it is not exposed to constant moisture.
It is tough to cut and watch out for the dust.
I would think you could build a frame and use the 1/2 type painted with a exterior coat and it would give you years of use at half the cost of plywood.
Do remember that each time you put a screw into it to hold your track down you are creating a hole for water to get in.