Is it a Popscicle stick? ... Is it a very skinny tombstone? ... Is it a home pregnancy test? No ... It's an Atlantic Coast Line concrete whistle post. Cast into this sign are depressions showing the traditional grade crossing whistle pattern - from the top, two longs, a short and a long. Time has passed, and this line in Mullins, South Carolina, has gone from ACL to SCL, SBD and, eventually, the Carolina Southern.