In January of 1943 Jack Delano spent a night in Calumet City, Illinois observing an Indiana Harbor Belt crew at work in the yard. A cooperative crew member demonstrated a few basic signals for Mr. Delano to document with his camera. The switchman is using a fusee (which most non-railroaders would know as a flare). These produce an intense red light, making it highly visible in the dark, especially during rain or snow.
There are several other signals used, but these are the three basic ones used for movement.
While these fusees are very bright and easy to see, it is perhaps more common for the switchman to use a lantern for his signaling, as it will burn for many hours and is more economical. But the fusee can’t be beat when conditions are really bad.