Quirks of London Underground vs. National Rail Mainline

LUL signals are similar in nature and purpose to the NL signals you may be familiar with. They ensure that a train can only enter a track circuit section (what NL calls a block) when the section is clear of other traffic and, if relevant, that a path is set and clear between the signal and the section it controls. There are a number of different types, each having a specific purpose and appearance and behavior. Note that any LUL signal capable of displaying a red aspect will have an accompanying trainstop to protect the section ahead.

LUL signals use red, yellow (NOT amber) and green aspects; with white being used for related Route Indicator (RI) lights. Different signal types can display 2 different colors specific to the type -- Red + Green for signals controlling a section, or Yellow + Green for repeater signals. Generally only one aspect will be displayed at a time, other than as Ed noted above when a signal changes from red to green while the trainstop lowers. Signals can also be located on the same pole or very close together, an example being a semi-auto signal above a related repeater signal.

Some signals display 4 colors, from top to bottom: Green, Red, Green, Yellow. These can display 3 different states:

Red + blank = Section occupied. Stop.

Green + yellow = This section clear, but following section busy so proceed with caution (expect a red at the following signal). This equates to an NL 3-light signal showing yellow.

Green + Green = this section and the following section clear (very useful info in dense areas so you know what's happening further ahead). This equates to an NL 3-light signal showing green.

This is the simplest type and controls entry to a track section, and defaults to a clear state or GREEN aspect. It will change to a red aspect when the front of a train passes the signal and enters its section. It will return to green (default state) when the rear of the train exits the section it is controlling - most typically passing the next signal in order. The Auto signal will have a simple "A" prefix designation of black letters on a white plate - e.g. A601. Auto signals are placed as needed to separate trains and maintain short head-ways (dense traffic).

This acts like an Auto but has a designation including the cabin code (see Semi-auto signal below) and an "X". This signal indicates that this is the last Auto signal before entering a controlled section protected by a Semi-auto signal. For example, the signal sequence approaching and leaving W Brom EB is: R689 (repeater), A689 (auto, home), ECX691 (starter, and last auto before controlled section, cabin code EC is Earls Court area), REC9A (repeater), EC9A (semi-auto, ECT approach multi-home). So the "X" signal warns that a semi-auto signal should be expected next.

These default to a RED aspect and generally protect an area requiring more caution - thus the default red. A typical example may be the approach to a converging junction, where the signal will only clear to green if the path is set for the train through the junction, and the section ahead is clear. The signal state and related aspect is evaluated as the train approaches - typically 1 or 2 blocks in advance - and if the route into the section is set and clear, the signal will change to green ready for the train to pass. The signal will revert to red once the front of train passes, and will remain at red (default state) once the rear of train exits the section, until it is re-evaluated when a subsequent train approaches. Semi-auto signals have a designation in black letters on a white plate showing the cabin code and a sequence number - e.g. WD39 (HAMS WB starter, cabin code WD).

Route indicators (also known colloquially on NL as "feathers" and on LUL as " 'arbor lights")  are used as part of semi-auto signals to show that a path is set in a specific direction at a diverging junction. There may be more than one path on either side of the RI, indicating specific paths to left or right. The RI will only light when the signal shows a green aspect. A good example is leaving the tunnel approaching HSK EB, where signal ED23 has 2 RIs on the right side. If RIs NOT lit this indicates the path is to Bay P4, if 45 deg right RI path is to Bay P3, if 90 deg right RI path is to P2 EB and onward around the upper Circle to ERD.

Repeaters are similar to, but not quite the same as NL Distant signals. Their purpose is to repeat the signal in advance (ahead) to provide advanced warning of its state. A yellow aspect means the following signal is currently at red. A green means the following signal is green. Repeater signals have an "R" prefix designation with a number the same as the signal number they are repeating, using black letters on a yellow plate - e.g. repeater R681 would precede A681 (auto), and REC9A would precede EC9A (semi).

A repeater may be deployed as part of a 4-light semi+repeater head, or a separate repeater-only head, or as a LUL-unique "fog repeater". Fog repeaters are small white rectangular signals and are used in areas that used to be prone to dense foggy conditions, and knowledge of upcoming signal state was critical information. They have an "FR" prefix designation plus the signal number they are repeating - e.g. FR681.

Shunt signals (color light or disc semaphore) generally control access on or off the mainline onto a siding or in/out of depot roads. We have also used color shunt signals in the various depots (Ealing Common, Lillie Bridge) as starter signals to enable controlled access from the depot roads onto the main line; and in exiting sidings (e.g. Parsons Green). They may also be co-located with a mainline signal which will display a red aspect if the shunt signal is clear - this indicating the path is NOT on or to the main line, but rather diverges to a siding or depot access road.    In those situations you will pass a main line signal at danger, obeying the cleared shunt signal, and we have prevented the tripcock from operating as you are making a legal controlled move. In some cases we have retained the old LUL semaphore disc shunt signals for historical reasons - example being entering ECMDT from Ealing Common EB. Note that in the route the color light shunt signals also have a number plate that has a unique number for reference that relates to the siding or depot road number, but these ddo not represent real world signal numbers.

There are a few other special signal types that we can not fully simulate as regards their exact real life operation. We have included them (and their correct numbering, indicating their purpose) in the route for completeness and simulated their behavior as closely as we can.

Speed control signals
These are used in areas where train speed needs to be reduced and verified before approaching a dense controlled area. A speed controlled signal will have an extra "0" suffix added to its expected sequence number. A typical example is the approach from Olympia (EC100) or W Ken (EC300) EB in the 20mph section entering the tunnel towards ECT. There are delta-track sections in rear of the signal that measure train speed through a timing section. The signal will only clear if the train speed is at or below the desired speed. As we can not accurately simulate this behavior, we have treated them as auto signals in the route and trust you will obey the associated speed limits.

Multi-home signals
Multi-homes are a way of further reducing train headway especially in areas of lower speed approaching a station. Typical behavior is as the train ahead leaves the station, signals in rear will clear in sequence allowing a following train to approach in a controlled manner much more closely to the train ahead leaving at higher speed. Multi-home signals are designated by black letters on a white plate, and have the same signal number but with an alphabetic suffix - e.g. the sequence running down the "slow Acton" bank EB into ACT is A581, WL103A, WL103B, WL103C, WL99 (outer home), WL109 (home), etc. As we cannot simulate this behavior exactly, we treat them as auto signals in the route; however observation shows they do clear successively in a similar manner to the real life signals.

So now you should be able to recognize the various signal types on the route and understand their purpose and behavior. Drive safe and enjoy!