How to tag a railway track-side equipment box/cabinet?

In Mexico we call them “caja” (box, very unspecific). Note it is big enough for a person to walk in.

You would think there would be something under railways=* but there doesn’t appear to be?

For similar cabinets roadside, you use Tag:man_made=street_cabinet - OpenStreetMap Wiki, so that may work here?

Alternatively, possibly something under Key:utility - OpenStreetMap Wiki?

My own answer: Use railway=signal_box, building=shed, material=metal.

From the wiki alone, it is not clear what tags to use. In OSM itself I found some of these boxes tagged with railway=signal_box. The other tags are my addition to make more clear what type of box is meant.

Is thar tag really used for utility cabinets? I’d have thought it was only for actual signal boxes.

Yes. It has many instances in USA, most of which are for walk-in railway cabinets.