Pere Marquette/C&O/Chessie used as a flanger sign a small white rectangle with two black circles mounted on an old boiler tube in advance of crossings and switches. Used on the lakeshore lines toward Muskegon and up north. Never saw them on the mains east of Grand Rapids. Like the one on the far left pictured here. https://forums.auran.com/threads/what-do-flanger-signs-mean-hi-res-images.159542/