The NPS has a "cultural resource document" that covers features at the Grant Kohrs Ranch near Deer Lodge, Montana. Clearly, this one is more complete than the one near Othello, but it probably looked similar at one time. Here is what the NPS says about the detector, pictured above, at the Grant Kohrs Ranch:

Quote:

National Park Service
A wheel flange detector is located just south of the railroad trestles, along side the Milwaukee railroad line. It consists of a gray metal utility box and concrete box with wooden lid. Two metal plates are also located on the inside edges of the adjacent tracks (see Photo 3-8-18). This system was installed to measure vibration caused by the trains, and is no longer operational.

You can read the whole document here:
[www.nps.gov]

The picture above is a screen capture from the .PDF, which is a large file. This is not the photo that "Brent" offered elsewhere in this thread.