Depends on the railroad, traffic, and track.
In the east on NS and CSX they are placed about every 15 to 20 miles apart. On the ex-B&O across northern Ohio they are located at mileposts 90.3, 73.3, 52.2, and 31.1; on the ex-B&O Toledo Sub they are at 178.8, 160.6, and 140.3.
Railroads with...