This is a re-railer for G scale model railways such as LGB, Bachmann etc.
I found that some cheap Chinese Nuquida coach copies were derailing on some of my inverse curves, so made this to try and get them back on track.
It uses four screws to hold and align the track. The screws I used were 3.5mm by 16, and these extend just below the re-railer bottom surface.

I have attached the 123D file so you can modify the design if your railway does not use LGB locos.