Not sure, my experience with this is in Australia, they’re all over the rail corridor but not on the sleepers as far as I know. There’s usually a specific phone number you call to alert, say emergency x3, your location and the nature of the emergency.

Just a stab in the dark but they may occur at regular intervals, and similar notation may be on the poles and other infrastructure. In the event of a rail emergency you give the operator that info and they will halt all trains that could be affected by the emergency