On both of my failures the crack was next to one of the mounting ears.
Thank you. Simple solution for me. Isolate the rail from vibration and stress. Make the mounting holes in the rail a bit larger, use some thin rubber between the four mounting points and the motor and use a thicker medium density foam "washer" between the rail and the tops of the mounting bolts. Throw a little high temp RTV on the threads of the mounting bolts and don't tighten much beyond "snug". Index the bolt heads with a Sharpie and check them after a few laps to make sure they are not backing out.