Great Western Express Signalling – A Brief Introduction
There are many intricate systems that train drivers have to master before being qualified to run out on the main, and none are more important than signalling.
In principal, signalling exists as a system used to regulate and direct traffic along the rail network, ensuring that trains are kept clear of each other at all times. The first timetables were a key step into the world of regulated rail travel, and as numbers grew for both passengers and freight, a plethora of varying systems were developed around the world, and today’s complex signalling makes trains one of the safest and most reliable methods of public transport to date.
The Great Western Main Line is equipped with the modern-standard British Signalling system, which has existed in its basic form since the days of the “Big Four”. As the GWML is being modernised for future Elizabeth Line services, the route is currently home to both old and new LED four aspect signals.
Unlike signalling in the US, which is predominately speed-based, UK signalling is typically route-based, where different signal aspects instruct the driver to either continue or brake accordingly for a cautionary, diversionary or stop aspect. All appropriate speed changes are handled exclusively by speed boards, not signals.
Below is the typical sequence of aspects when approaching another train.
- Green Aspect: Clear, proceed normally at Permissible speed*
- Double Yellow Aspect: Preliminary Caution, proceed expecting Caution at next signal
- Yellow Aspect: Caution, proceed and preparing to stop at next signal
- Red Aspect: Danger, stop.
*Permissible speed: the maximum speed of a certain section of railway line. Permissible speed indicators denote changes of the Permissible speed, the location of these indicators, and what they say, is information train crew are expected to know as part of their route knowledge.
A Signal outside Reading station, Signal No. T 1698 to be precise, displays a clear aspect (above), allowing the Class 166 driver to pull away. After they have done so, the signal will drop back to a danger aspect (below).
These four are the most typical signal aspects you will find on approach to an occupied platform or junction. It is standard practise to start braking as soon as you see the first cautionary signal (minimum of step 2 braking in the Class 166, step 3 or 4 in the HST, and whatever is required for the weight of a consist in the Class 66). You must never approach a stop aspect at more than 20 mph, doing so can trip the emegency brakes.
In the real world, drivers are expected to know the location of signals, and how they need to brake accordingly to come to a safe stop. Factors can range from train weight, distance between signals and inclement weather, so knowing your train and route is very important.
This type of signalling is called ‘track circuit block signalling’, only allowing one train to be in a block at any one time. If a train is standing on a section of track, it is completing that circuit and the signal immediately behind the train is set to red, the one behind that is yellow and so on. As the train moves forward, the signalling for any train behind clears to the next aspect.
A demonstration of the typical aspect sequence after a train has passed the signal in question, which in this case, is T 483 which stands on the Down Relief line at Iver station.
In order to conserve momentum, especially in heavy trains that can take a lot of power to get going, it is recommended to employ defensive driving techniques when approaching a stop aspect. The trick is to crawl towards the signal for as long as possible, very gradually decreasing your speed in the hope that the signal changes to a proceed aspect, before having to come to a complete stop.
Another form of defensive driving is known as ‘chasing yellows’, this is where you approach a yellow or double yellow aspect and lower your speed enough to match the service ahead of you, consequently the signals change periodically as you approach them. While this can aid in keeping speed up and staying on schedule, it is important to remain vigilant; if the service ahead of you stops unexpectedly, you need to be able to still stop before the upcoming danger aspect.
Feathers and Route Indicators
The next step in learning the signalling is junction (or diversionary) signals. If the signaller has pathed you along a different route from which you are driving, a diversion, the signals on approach to the junction will forewarn you of the upcoming route change.
- Green Aspect: Clear, proceed normally at Permissible speed
- Flashing Double Yellow Aspect: Preliminary Caution, proceed initialising brake towards upcoming speed restriction as indicated
- Flashing Yellow Aspect: Caution, proceed, continue to brake towards speed indicated, expect feathered aspect at next signal
- Feathered Aspect: Clear/Caution, proceed expecting diversion at next junction as indicated by feather, drive in accordance with relevant signal aspect
This feathered route indicator tells the driver that they are diverging to the left under a double yellow ‘caution’ signal. Note that there are multiple possible feathers in some instances, each one corresponds with a different diverging route option. If no feather is lit, no diverging is expected.
The sequence of flashing yellow aspects indicates that a diversionary route is upcoming, tied with this is usually a speed board telling you what the speed will be at that junction, and so a brake application is required. At the end of the sequence, the feathered signal will display a series of white lights, these tell whether you are turning left or right (graphic provided demonstrates a left-hand turn). If you were pathed to continue, and not divert, these white ‘feathers’ will not be illuminated.
The feathered aspect will be accompanied by a proceed aspect, if this were yellow, it means that the next signal is red and you must prepare to stop, but it could also show green indicating that you are clear to continue on.
Another piece of information that can be given to drivers is through a ‘theatre’ route indicator. At places such as London Paddington, where there are multiple tracks and routes, the starting signal theatre can inform the driver which route they are pathed on such as ‘1’ for Track 1 etc. or on approach to a terminus, which of the many platforms you are cleared into.
A prime example of a ‘theatre’ route indicator, seen here on SN 5 at London Paddington. The Green aspect denotes clear, and the ‘1’ tells the driver which of the lines they will be routed down. This information is very important, there are so many lines out of Paddington that spotting which signal is yours can be a challenge – knowing your path can let you know where to look down the line.
It is common for a drivers’ view of a main signal to be obstructed, be that because of where they stopped on the platform, or a sharp curve along the route. In these cases, banner repeaters are used to tell the driver what aspect the next signal is displaying. There are three states that modern banner repeaters can take:
- White Horizontal Banner: On, the signal it applies to is set at danger
- White Diagonal Banner: Off, the signal it applies to is set to a proceed aspect
- Green Banner: Off, the signal it applies to is set to specifically a green aspect
Two banner repeaters seen here at Ealing Broadway, both displaying a clear aspect, as correspondent with their respective signals. Both SN 2092 BR and SN 2091 BR (BR = Banner Repeater) are green, indicating that the paths beyond both SN 2092 and SN 2091 are clear.
Before the introduction of the green banner, a diagonal white banner could mean any proceed aspect from green to single yellow, but the added indication of a fully clear aspect allows drivers to pull away and know they can accelerate to Permissible speed.
Position (or Shunting) signals allow trains to move forward into potentially occupied sections, and are most commonly found guarding platforms, sidings and depot lines. They differ greatly from main signals, particularly as in a track circuit block, drivers know whether the next block is free or occupied, but when passing a Position signal, the section must be treated as occupied. This allows for the coupling of trains which would normally be limited to their own blocks.
There are a few types of Position signals:
- Subsidiary Signal: used in conjunction with a main signal for calling-on procedures
- Ground Position Light: typically found on standalone posts with no corresponding main signal
- Limit of Shunt Signal: a type of ground position light which can display ‘stop’ only
Two examples of ground position signals at Acton sidings, both are displaying a stop aspect. Note however that they are not specifically ‘limit of shunt’ signals, as they both have the capabilities of displaying other aspects.
Subsidiary signals display only a single aspect, two white diagonal lights. When lit "1A", the driver has permission to pass the main signal regardless of its aspect. When the subsidiary signal is unlit "1B", drivers must obey the aspect displayed by the corresponding signal. See, ‘Permissive Signals’ below.
Ground position lights can display either two white diagonal lights "2A", indicating that shunting movements are permitted, horizontal white and red lights "2B" or two horizontal red lights "2C" that indicate stop, this is the furthest which a shunting train is allowed to proceed. Certain ground position lights may be fitted with yellow bulbs instead of red, these would apply only to movements in the direction to which the signal can be cleared, other movements can pass at caution at any time.
Limit of Shunt signals "3" are a form of ground position light that is fixed to ‘on’, with no space for a proceed aspect, meaning all movements are not to pass this point unless authorised by the signaller.
Permissive signals are used when certain movements, that defy the block signalling method, are required. For example, if two separate DMUs are coupling away from the depot, such as at a station, or a locomotive needs to rescue another service, they must be allowed permission to enter an occupied block in order to complete their duties. This is the most common permissive signal procedure, and is governed by ‘Calling-on signals’.
Extreme caution must be adhered to, drivers should travel at speeds where stopping at a short distance is next to no issue, and under no circumstances should this need to exceed 15 mph.
T 1685 is displaying a ‘calling-on’ aspect; the block ahead is occupied, so the signal stays at red, yet the two white lights indicate permission from the signaller to proceed with extreme caution. Additionally, this signal is also fitted with a ‘theatre’ route indicator, telling they driver they’re going into Platform 9 at Reading station in this instance. Note that T 1687 is displaying a clear feathered aspect, while T 1689 is set at danger.
There is a lot of information in here, but all of it will we useful at some point when working the Great Western Main Line, so take the time to learn it if necessary, or just refresh your memory. Head to the Store and put your newfound knowledge to the test, as Train Sim World: Great Western Express is available now! ■