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The signal's aspect is based on a combination of the aspects each individual head displays. Where a signal has multiple heads, aspects are read from top to bottom and are described as "X over Y over Z". Dwarf signals are smaller signals used in low-speed or restricted-clearance areas. Most signaling aspect systems have a parallel set of aspects ...
Signal aspect and indication illustrations instead appear in each railroad's system special instructions or operating timetable for the region or division where the aspects and indications apply. This practice is necessary due to the lack of uniformity in aspects between the multitude of railroads participating in GCOR, which includes a number ...
The signal head is the portion of a colour light signal which displays the aspects. To display a larger number of indications, a single signal might have multiple signal heads. Some systems used a single head coupled with auxiliary lights to modify the basic aspect. Colour light signals come in two forms.
Unlike ordinary signals, there may be a series of identical signals installed along the track so that at least one is always visible to the train driver at any time. All signals in the same group display the same indication simultaneously. A 'stop' indication means "stop immediately", even if the train is not at the signal. The signals can also ...
The signal prior to the junction signal will now show a single flashing yellow aspect and the signal prior to that one will display two flashing yellow aspects. The driver's route knowledge tells them permissible speed across the diverging junction, and they will begin to slow the train upon seeing the two flashing yellows.
The vertical position indicates a "clear" aspect. Lower-quadrant stop signals at St. Erth in 2007. British semaphores come in lower-quadrant and upper-quadrant forms. In a lower-quadrant signal, the arm pivots downwards for the less restrictive (known as "off") indication. Upper-quadrant signals, as the name implies, pivot the arm upward for "off".
The CNW went further and eliminated the wayside intermediate signals in the stretch of track between Elmhurst and West Chicago, requiring trains to proceed solely based on the 2-aspect cab signals. The Chicago, Milwaukee, St. Paul and Pacific Railroad had a 3-aspect system operating by 1935 between Portage, Wisconsin and Minneapolis, Minnesota. [7]
Two aspect signalling also makes use of the "Warning", "Reduce to Medium Speed" and "Proceed" repeaters mentioned in the Three aspect section, these signals being primarily used on the border between two and three position signal areas, to give drivers an indication of the aspect of the next three position signal.