Page 10 - November December 2009

Signalling the Modular Way
Introduction/Background
The “Worcester N Gaugers Group” operate an N scale modular layout (closely based on the oNeTrack standard for N scale) using Digitrax as the preferred DCC system for all operations. My interest in signalling stems back many years. As a child when living in my home town (city) of Preston I always took a keen interest in railway operations around Preston, especially before the era of “power boxes” and “electrification”. In more recent times I held the positions of signalman on the Severn Valley and assistant signalling inspector on the Gloucestershire & Warwick- shire Railway (1994–2008).
The interest in all things signalling led me to consider installing signalling on my North American modules. One of our members had already led the way on his modules, and now it was my turn to make a leap into the unknown.
A brief introduction to the prototype
Interlockings and Traffic Control System
A Traffic Control System (TCS) on a railroad combines groups of interlockings with an Automatic Permissive Block System (APB) and Traffic Locking.
Traffic locking locks the direction of travel in a track when an interlocking clears a signal to that track, and it prevents the interlocking in the other end of the track from sending trains onto that track. This is particularly relevant when operating trains over single or multiple lines where trains
can run in either direction.
APB is the most common form of block signalling used in single-track areas. The name APB derives from the absolute protection for opposing movements, with permissive (Stop
and Proceed) protection for following movements.
Traffic locking also locks all opposing signals to their most restrictive aspect, and blocks adjacent interlockings from clearing a signal towards that track. Traffic locking stays in place as long as there is a signal cleared to that track, and as
long as the track is occupied by a train.
TCS rules are often referred to as Centralized Traffic Control (CTC) rules, but the correct term is TCS.
Centralized Traffic Control
Centralized Traffic Control (CTC) is a term used to describe a system that allows remote control of TCS. CTC allows a single dispatcher to directly control and monitor long sections of railroad or even a sub-division (very much like our UK signalling centres). CTC provides the greatest traffic capacity for a line, as the person with the traffic overview is also the person directly controlling the traffic.
CTC controls groups of interlocking plants and ABS (signals that are automatically operated by track circuit occupancy on plain sections of track) with traffic locking as in TCS, with an interlocking often referred to as a Control Point (CP). CTC uses the rules of operation for running trains on signal indication alone.
The essential to achieve this is the presence of a Traffic Control System (usually in the form of interlocking plants). These inter- locking plants are then accessed remotely through a cable/wire- less network to the main CTC system which is controlled by the Dispatcher (signaller/operations manager in UK terminology). North American versus UK signalling practice
The major difference between US and UK practice is that in the USA, signals indicate the speed at which a train must operate at based on signal indication. In the UK, signals indicate the route that the train will take (i.e. the driver must have the route knowl- edge regarding speed limits for the indicated routes). Another major difference is that all block signals in the UK are “Absolute”, i.e. when the aspect is red, the train must stop and wait until the signal indicates a less restrictive aspect. In the US, block signals
can be a mixture of “absolute” (usually identified by the absence of a number on the signal post) and permissive (stop and pro- ceed), even though the signal aspect is red (always identified by a number located on the signal post).
Choosing an appropriate operating scenario for my modules
When assembled, my modules comprise of 16 ft of single main line, and a 13ft siding (passing loop in UK parlance) with indus- trial leads coming off the siding at one end to serve a paper mill. The signalling project would put in place a Traffic Control Sys- tem (TCS)/Centralized Traffic Control (CTC) to dispatch the main line and siding. However, access to the paper mill would be under local control but interlocked with the traffic control system.
The CTC panel would be designed to remotely control the switches connecting the siding to the main line. Signals would be placed in the appropriate locations to control traffic to and from the siding/main line.
Furthermore, a local switching panel would also be provided to operate the access to the paper mill.
A diagram of the CTC controlled area is shown below:
Richard Czaja (Worcester ‘N’ Gaugers Group and British Region NMRA representative (#127428) for Three Counties Sub-Division)
• Note 1: the black track area is track circuited (“block occupancy” sensors fitted).
• Note 2: breaks in the track indicate individual track circuit block sections.
• Note 3: Items controlled by the interlocking plant and CTC panel are shown in “boxes”.
Selecting the signal control system and signals
Control system
In line with one of our other group members’ signalling system, I choose to use the products manufactured by CML Electronics Ltd, a UK-based supplier who designs and builds specific products to control switches (DAC10 – 1 off ), signal controls and interlocking with inbuilt TCS logic (SIGM20 – 1 off) and CTC logic (DTM30 – 1 off). The block occupancy detectors are manufactured by Digitrax (BD4 – 2 off). The biggest benefit of using these products is that they all use the Digitrax Loconet® to communicate to each other – not dissimilar to the prototype using an adapted form of TCP/IP “Ethernet” networking. DAC10 board
The DAC10 accessory decoder allows control of up to eight solenoid or Tortoise-type point motors on a DCC-controlled model railway layout. The DAC10 also has provision for local switches and LEDs for manual operation and status indication. Its switch inputs can also be used for position feedback or as sensor inputs, using the Digitrax Loconet® to report status to the CTC panel (DTM30).
SIGM20 board
The SIGM20 is an automatic controller for eight or ten colour light signals. The signals are controlled automatically in response to the settings of the track and the passage of trains. This allows
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