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

You are thinking of dried Sea Sponge, you can get them in pharmacies and cosmetics stores where I live