Signal Configuration Overview

Table of contents
  1. Signal Configuration Overview
    1. Introduction
    2. Terminology
    3. Signal Mast Aspect Usage Examples
    4. Signal Mast Aspect Configuration Summary

Introduction

In this section, we delve into the intricacies of signal planning, focusing on the configuration of signal aspects rather than the physical setup of signal masts and lamps. Here, we outline various configurations of signal mast heads and lamps and detail the downstream elements that govern signal aspects.

By focusing on signal aspects and the logic underlying their configuration, this guide aims to equip you with the tools necessary for effective signal planning on your model railroad.

Setting signal aspects is configured by define one or more logics, which check for downstream conditions and perform actions to set the signal aspects. During planning, we show a logic statement using the following form:

<Logic statements follow the format: IF (conditional) THEN action(s) for true conditions ELSE actions for false conditions>

Multiple logic statements are grouped together into Signal Group to support complex configurations; multiple headed signals and when checking blocks, main routes, divergent routes, and other downstream signals.

Terminology

Below is a set of terms used in both planning and configuring of signal aspects:

  • Logic Statement: for planning purposes, a logic statement is in the form of an typical if-then-else statements as shown here:

    IF conditional THEN action(s) for true conditions ELSE actions for false conditions

    Example: 1. If Block is Occupied, Then Set aspect to Stop and Exit Else Set aspect to Clear and Continue.

    Explanation:

    - `1.`: indicates this is the first logic statement in the group.
- `Block is Occupied`: is the conditional, consisting of a single variable to be check for true or false.  Up to two variables can be specified, seperated by a logic operator like `OR`,  and `AND`
- `Set aspect to Stop`  is the action that to be performed when the conditional is true.
- `Continue` determines that the processing of additonal logics should continue.  If this is the last (or only) logic statement in the group, then processing of the group will stop and proceed with the next logic group.
- `Set aspect to Clear` is the action to be performed when the conditional is false.  

  • Conditionals: condition(s) to be evaluated, resulting in either true or false. Conditionals contain either one or two variables. When two variables are define, a logic operator is used; V1 and V2, V1 OR V2, V1 Only, etc.

  • Actions: is what happens when the conditional is true or false. Typically the action sets the aspect or does nothing.

  • Logic Group: represents a collection of logic configurations accessible via the CDI tool in the Logics and Conditionals section.

    • Planning example:
      1. If Block is Occupied, Then Set aspect to Stop and Exit Else Continue.
      2. If Downstream Mast’s Track Circuit indicates Stop, Then Set aspect to Approach and Continue Else Set aspect to Clearand Continue.

  • Logic Processing determines what happens after the a logic statement is processed, Exit to stop processing the logic statements in the group, or Continue to continue with the next logic statement (e.g. additional conditions need to be evaluated and aspects set). Note that processing the last logic statement in the group will automatically exit the group upon completion.

  • Track Circuit: Track Circuits report the downstream track speed as shown by a mast linked to the track, simplifying the number of conditions that need checking and are particularly useful when dealing with downstream masts. During configuration, a track circuit is used as a variable in the conditions. For example, when the downstream signal aspect is reporting a stop speed, the current mast typically would show an approach speed.

    • Planning example: If Downstream Mast’s Track Circuit indicates Stop, Then Set aspect to Approach and Continue Else Set aspect to Clearand Continue.

Signal Mast Aspect Usage Examples

Aspects Mast Name Signal Type Purpose/Use Example Triggers
Caution, Clear Mainline Distance Signal Two-Aspect Signal Provide advance warning of mainline signal status. Next Block Occupied or Clear
Stop Yard Entry Mast Single-Aspect Signal Signal yard entry restrictions. Yard Block Occupied
Stop, Approach, Clear Crossover Signal Mast 3 Three-Aspect Signal Indicate safe crossover alignment. Turnout Aligned or Block Status
Stop, Clear Mainline Signal Mast 1 Two-Aspect Signal Control train movement on a mainline. Block Occupied or Turnout Misaligned
Stop, Clear Yard Exit Mast 6 Two-Aspect Signal Signal safe yard exit onto mainline. Mainline Block Status
Stop, Diverging Clear, Clear Junction Mast A Three-Aspect Signal Control train movement at a turnout junction. Turnout Alignment and Block Status
Stop, Restrict, Clear Siding Signal Mast 2 Three-Aspect Signal Indicate siding availability. Siding and Mainline Block Status
Stop, Restrict, Approach, Advance Approach, Clear Mainline Mast 4 Multi-Aspect Signal Provide multiple aspects for mainline operations. Turnout and Block Status

Signal Mast Aspect Configuration Summary

The following table is provided to assist in simplifying the configuration of signal mast aspects. The configurations are listed based on the number of signal aspects to be set (heads and lamps) and what is influences the aspect downstream of the signal (blocks, turnouts, masts).

To further simplify the planning:

  1. only three aspects are define; Stop, Approach, and Clear.
  2. check for all conditions that set the aspect to Stop, followed by conditions for Approach, and finally Clear.
  3. even thought configuring aspects requires configuring the indications, the setting of the lamps is not defined below since that varies by the aspect rules and signal head type.
  4. utilize the minimum necessary number of logic statements.
  5. track circuits are used to check for downstream track speeds

Note that up to 4 actions can be executed for true and false conditions, allowing for a tumble-down to be configured for setting up to 4 aspects. For example, when configuring the first signal located at the beginning of a set of blocks between sidings (headblock), configure multiple actions for this signal where each action sets the same aspect downstream signals, thus creating a tumple-down of the signals being set (all appear the same).

The use of Exit and Continue within the logic statements’ actions signals whether the logic processing for that group should cease (‘Exit’) or proceed to evaluate the next logic statement (‘Continue’). Generally, once the appropriate aspect is displayed based on the evaluated conditions, the logic processing for that group concludes.

Ordering Conditionals: When crafting a logic statement for a signal, it is standard practice to first assess conditions that necessitate the most restrictive track speed (e.g., Stop), followed by conditions for intermediate speeds (e.g., Approach), and lastly, conditions allowing the least restrictive speeds (e.g., Clear).

Signal Mast Configuration Downstream Elements Logic Group (one or more logic statements) Visual
Single 2-Lamp Head 1 Block, 1 Masts, 0 Turnouts 1. If Block is Occupied, Then Set aspect to Stop and Continue Else Set aspect to Clear and Continue.
Single 3-Lamp Head 1 Block, 2 Mast, 0 Turnouts 1. If Block is Occupied, Then Set aspect to Stop and ExitElse Continue.
2. If Downstream Mast shows Stop, Then Set aspect to Approach and Continue Else Set aspect to Clearand Continue.
3-Lamp Head over 2-Lamp Head 3 Blocks, 3 Masts, 1 Turnout 1. If Turnout Block is Occupied, Then Set Upper Head aspect to Stop, Lower Head aspect to Stop, and Exit Else Continue.
2. If Turnout is Thrown, Then Set Upper Head to Stop and Continue Else Continue
3. If Downstream Mast shows NOTClear Then Set Upper Head aspect to Approach and Continue Else Set Upper Head aspect to Clear and Continue.
4. If Turnout is Closed OR Divergent Mast shows NOT Clear, Then Set Lower Head aspect to Stop and Continue Else Set Lower Head aspect to Clear and Continue
3-Lamp Head over 3-Lamp Head 2 Blocks, 2 Masts, 1 Turnout 1. If Turnout is Diverging, Then Upper Head Red, Lower Head Green for Diverging Route;
2. If First Block is Occupied, Then Upper Head Red, Lower Head Yellow;
3. Else Upper Head Green, Lower Head Green;		  
2-Lamp Head over 2-Lamp Head 1 Block, 1 Mast, 1 Turnout (Diverging) 1. If Turnout is Diverging, Then Upper Head Red, Lower Head Green;
2. If Block is Occupied, Then Both Heads Red;
3. Else Both Heads Green;		  
Twin 3-Lamp Heads (Side by Side) 2 Blocks, 2 Masts, 2 Turnouts 1. If Either Turnout is Diverging, Then Corresponding Head Shows Yellow;
2. If Either Block is Occupied, Then Corresponding Head Shows Red;
3. Else Both Heads Show Green;		  
2-Lamp Head over 3-Lamp Head 1 Block, 2 Masts, 1 Turnout 1. If Turnout is Diverging, Then Upper Head Green, Lower Head Yellow;
2. If Block is Occupied, Then Upper Head Red, Lower Head Red;
3. Else Upper Head Green, Lower Head Green;		  
Dwarf Signal, 3-Lamp 1 Block, 0 Masts, 1 Turnout (Diverging) 1. If Turnout is Diverging And Block is Occupied, Then Display Red;
2. If Turnout is Diverging, Then Display Yellow;
3. Else Display Green;		  
Vertical Stack of 3-Lamp Heads 3 Blocks, 3 Masts, Multiple Turnouts 1. If Any Turnout is Diverging, Then Corresponding Head Red;
2. If Any Block is Occupied, Then Corresponding Head Yellow;
3. Else All Heads Green;		  
4-Lamp Head (Single or Multiple) Specialty areas like speed-controlled zones 1. If Speed Restriction in Place, Then Display Aspect According to Restriction;
2. If Block Ahead is Occupied, Then Display Yellow;
3. Else Display Green;		  
Last updated on: May 10, 2025 © 2025 Pat Fleming