Signalling Styles

Signalling Styles

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I would like to acknowledge our Danish (I think) friend who suggested the 4 signalling styles, if only I could remember who it was that suggested this scheme. Please speak up.

The overwhelming majority of signals provide occupancy information of some sort. In addition, either speed information, or route information, or both, is provided as well. Occupancy information is not discussed further in this document.

I know of no signals that do not provide occupancy information, yet provide anything besides very restrictive (or no) speed information. If anyone knows of such signals, please let me know.

I've expanded the original 4 classifications to accommodate certain N. American railroads whose signals provide both speed and route information. I simply call this category "mixed". Such signals always seem to provide route information first (by proximity and time) and speed information second (e.g., Diverging Approach Slow). Are there any that do the reverse (e.g., Medium Approach Diverging)?

Finally, note that these classifications would apply to an individual railroad as a rule, not as an exception. For example, Amtrak, one of the most strongly speed-signalled railroads around, has 2 locations where strong route signals are installed - but these are an additional, exceptional installation noted as a "Signal[s] not in Conformity with Typical Aspects".

"Strong" route signalling
The signal provides exact or nearly exact route information, with no speed information. The route is specifically and explicitly provided to the engineer, e.g. by so-called feathers on the signal, or a track number display. The engineer must know, from another source, the allowed speed for the lined route. The granularity of speed limits may be quite fine, e.g., one switch may have a speed limit of 20 MPH, the next one 25 MPH. Example: British Rail.
"Weak" route signalling
The signal provides only nominal route information. In the limiting case, specific route information is known. [The limiting case consists of a location where there is exactly one straight route and one curved route. The speed for the given route must, theoretically, be known and/or found in the Timetable/Special Instructions, but generally, all diverging routes within the given interlocking are of the same speed. Generally, there is extremely little variability in the speed of diverging routes among different interlockings. Example: most western US roads.
Mixed route and speed signalling
Some signals provide route information. This is usually nominal route information (e.g. "Diverging Clear"), not specific route information as in the strong route signalling case. Route information may be specific in the limiting case. Other signals of the same railroad clearly provide speed information without route information. Example: Norfolk & Western; Southern Rwy.
"Weak" speed signalling
The signal provides nominal speed information. However, the lack of a large number (> 2) of possible routes at most interlockings effectively provides route infomation. Example: Baltimore & Ohio (mostly double track).
"Strong" speed signalling
Signal provides occupancy and speed information only. Route information (track numbers, e.g.) not provided, even in large terminal areas. The large number of routes at many locations prevents exact knowledge of the route, and when such knowledge is considered essential, additional provisions are made for providing route information. Example: Pennsylvania, New York Central (former used a lighted arrow alongside the interlocking signal to indicate route in (2) selected locations, now increased to 3 by Amtrak).

Mark D. Bej
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