Failuresaurus Trails are automatically created by Failuresaurus, when they walk over blocks. If left for too long, a free Failuresaurus could easily cover ones base in this trail. If it is broken with any item it will have a chance to drop slime balls.
The placement works like a vanilla rail, and might not look like it fully connects all the time (see the picture). It is lime green like the main Failuresaurus design. It was added in the 7.1 Update, along with several other Failuresaurus changes.