North American Railroad Signals - Common Signal Rule Classes

Common Signal Rule Classes

Most North American railroads have between 10 to 20 separate signal rules, each which are often represented by multiple aspects. However, all of these complicated rules revolve around the simple premise of informing the locomotive engineers how they are to operate their train in the present location, and what they are to expect at the next signal location. From here the large set of rules and aspects can be broken down into a small number of classes which are common to all North American signaling systems.

• Automatic Block - Block aspects convey basic track occupancy information and advise the engineer (operator) which of the basic signal rules (common to all railroads) he/she is to follow in the operation of his/her train at any point on the railway line. These include Clear, Advance Approach and Approach which instruct the engineer to "expect no stop", "expect stop at second signal" and "expect stop at next signal" respectively. Advance approach is only used in situation with short signal blocks to ensure trains have enough stopping distance. These are the most common signal aspects in North America and are the only aspects most automatic block signals need to display.

• Approach at Speed - When a train needs to be told to slow down due to dynamic conditions an "Approach Speed" aspect is used. These inform the engineer to slow to a prescribed speed by the next signal. The most common reason for this is that the train is to take a diverging, or non-Normal speed route at the next interlocking. Signals of this type include Approach Medium, Approach Limited, Approach Slow and Approach Diverging. These signals are typically displayed on the distant signal to an interlocking, but can sometimes be used with short signal blocks in place of Advance Approach.

• Diverge to Clear - This class appears only on absolute signals and informs the engineer that the train will be taking a diverging route and need not expect a stop at the next signal. In speed signaling the engineer is informed of the speed the train needs to take the route at, in weak route signaling the engineer is just informed of a diverging route. Signals in this class include Medium Clear, Slow Clear, Limited Clear and Diverging Clear.

• Diverge to Stop - Same as above only the train can expect to stop at the signal after the interlocking. These signals include Medium Approach, Slow Approach and Diverging Approach.

• Combination Signals - These combine functions of a "Diverge to" signal with an "Approach Speed" signal and occur in areas of complex trackwork where there are no intermediate signals between one interlocking and the next. In the United States only a few combination signals like Medium Approach Medium, Medium Approach Slow and Diverging Approach Medium/Slow are ever found in rulebooks and not frequently used in practice. The Canadian standard rulebook contains signal rules and aspects for every possible combination.

• Restricted Speed Signal - This class of signals is displayed for trains moving into a block where a track circuit has been de-energized or does not exist. A "shunted" track circuit indicates either the block is occupied by another train or railcar, or there is a problem such as a broken rail or flooded track. Where a track is not protected by track circuits that track must be presumed to be occupied. As the name implies this signal requires trains to move at Restricted speed, specifically with the ability to stop short of an obstruction. Restricted speed signals take many forms including Restricted and Restricted Proceed where trains must simply pass the signal at restricted speed and also Stop and Proceed, where a train must come to a complete stop before proceeding at restricted speed. Stop and Proceed has fallen out of favor with most freight railroads due to the fuel and time savings of allowing the trains to not come to a complete stop. This aspect class can be displayed on almost all railroads in North America.

• Stop Signal - Stop signals are displayed on Absolute signals, in fact the ability to display an absolute Stop is part of that signal type's definition. Stop is the most important signal as passing a signal at Stop presents a serious risk of accident. Engineers committing a Stop signal violation automatically have their Federal certification suspended and are frequently fired. Stop signals can only be passed upon special permission from a control authority.

• Cab Signaling Signals - Where cab signaling is employed without fixed trackside automatic signals, special signal aspects are required at absolute signals. These include some sort of absolute block "Super Clear" signal that allows passage to the next interlocking with a fixed signal and also the "Cab Speed" signal that informs the engineer to proceed under direction of cab signals.