This circuit was designed by Stan2004 and is documented in the Electrical Reference Materials & Manuals sticky topic above in this category. It is item #1 in the topic in the thread called "Electronic Projects on the OGR Forum Developed by OGR Members", graciously maintained by rtr12. The thread has links to the original topic, which contains the circuit and the original gerbers.
Recently I wanted to put a tester in a caboose cupola roof, but the original board size a little large. So I redesigned it to about 3/4" x 1.125" size, which fits way better. Here is a 3D board view and a few pics of the finished caboose:
Board ready to go into the cupola. Note the leds are on the other side (top) of the board:
Mounted up inside the cupola, and re-installed on the shell:
Finished and on the track being tested:
The green led indicates the board is powered up. If a spike higher than 33 volts is detected as the car rolls along, either one or both of the red leds flickers unmistakeably.
I used 3mm leds to minimize the impact in the caboose, but it is still visible enough for track testing purposes. All parts are the same as the original; the only change is the board itself. If anyone wants to build one like this the gerbers are attached 2 posts down.