Don't forget to bring food and a light for the tunnels.
This is absolutely not recommended in the winter. The second section is going to be blocked by snow. Every other season is great.
A flat, unpaved road crossing some very interesting area. The first part of the railway runs under the shade of old woods before ending at the Gaggio Reservoir. You'll need to follow the road that goes around the reservoir and then goes back down to the connection with the second section of the rail trail. Ignore the signs telling you that the trail is closed, they are there just as a disclaimer by the electric company who owns the property (if something happens to you, they don't want to be liable).
The second section of the rail trail is wilder and more exposed to the sun. Also, it goes through a few very dark, but short, tunnels. You are going to need a light for the two longest tunnels. The experience of riding on the gravel in the dark is incredibly fun!
Though it may show on maps, the section of the rail trail east of the starting point to San Matteo appears to be abandoned and not safe. To reach San Matteo, it's much better to take the gravel road from Arigna. This part of the Arigna Valley is extremely steep.