North American Railroad Signals - Signaling Aspect Systems

Signaling Aspect Systems

There are two main types of signaling aspect systems found in North America, Speed Signaling and Weak Route Signaling. Speed signaling transmits information regarding how fast the train must be going in the upcoming segment of track, weak route signaling transmits information related to the route a train will be taking through a junction and it is incumbent upon the engineer to govern the train's speed accordingly. Weak Route Signaling is applied with the term "Weak" because some speed signal aspects may be used in the system and also because exact route information is not typically conveyed, only the fact of a diverging or straight route, each having a predictable range of known speeds.

Typically railroads in the Eastern United States ran speed signaling, while railroads in the west used route signaling, with some mixing of systems in the Midwest and South. This was due to the lower train density in the west combined with generally simpler track layouts. Over time, the route signaling railroads have incorporated segments of speed signaling through merger and have also adopted more speed based aspects into their systems. Of the five major Class 1 railroads in the United States, CSX uses speed signaling, Union Pacific and BNSF use speed enhanced route signaling and Norfolk Southern uses a mix of speed and route signaling based on the original owner of the line. Commuter railroads and Amtrak all use speed signaling where they own or maintain the tracks they run on. Canadian railroads all use a strong system of speed signaling in Canada, but have some segments of route signaling on lines they have acquired in the United States.