Badger's Workbench

IRSE Mod A 01 - Introduction To Railway Signalling

Some notes on IRSE Introduction to Railway Signalling.

Principles and Functions of a Signalling System

Principles

A line is divided into block sections, and only one train is allowed per block, except in special circumstances. Entry to, and exit from, each block is controlled.

Functions

4 safety functions:

  1. Prevent routing conflicts
  2. Maintain safe train separation
  3. Protect from driver error/incapacity
  4. Supervise maximum speeds

Non-safety: efficiency, regulation, information.

Generations of Signalling Systems in the UK

Mechanical Signalling

Originated in mid 1800s, gradually replaced in some areas 1930s onwards, still in use. Points are operated via rods, and signals are operated via wires, driven manually by the signaller using large levers. 2 types of main signal:

Multiple Aspect Signalling

First installations in 1930s, widely rolled out in 1960s-80s, still being installed. Functions of stop and distant signals are combined into a single signal, usually with 3 or 4 main aspects.

The 4 main aspects are:

The first cautionary aspect seen by a driver must be positioned at least one train braking distance from its red. So signals must be at least 1 braking distance apart for 3 aspect signals, or 0.5 braking distances apart for 4 aspect signals.

Cab Signalling

Aims to replace lineside signals, instructions to the driver are displayed in the cab. Instructions include:

The train knows its precise position, and reports this via a secure high reliability radio link. E.g. ETCS.

Components of a Signalling System

Train Detection

Track Circuits

Comprises a transmitter (battery) at one end of the section, and a detector (e.g. relay) at the other. Frequency Shift Keying (FSK) and coded audio types are immune to electromagnetic interference (EMI) from electric trains. Designed to fail safe: any break in the circuit has same effect as a train present.

Axle Counters

Avoids the need for insulated rail joints (IRJs). Immune to EMI. Can be remotely restored.

Point Operating Mechanisms

3 functions:

  1. Move the switch rails.
  2. Lock them in position.
  3. Detect the switch rail position and lock state.

Mechanical Points

Points operated by levers via rodding. Movement and locking initially separate. Detection by position of rodding.

Point Machines

Combine all 3 functions in 1 machine. Usually electric but some pneumatic.

Signals

Semaphore or colour light. There are many subtypes - route indicators, shunt signals, etc. Incandescent bulbs are gradually being replaced with arrays of LEDs.

Interlocking

3 functions:

  1. Monitor train positions in the control area.
  2. Maintain signalling outputs (signal aspects, point states, etc.) in the control area.
  3. Process the signaller's inputs, validate their safety, and set outputs accordingly.

Mechanical Interlocking

Comprises steel bars and tappets arranged to prevent levers being operated in unsafe combinations. Places some reliance on the signaller for safety.

Relay Interlocking

Developed from 1930s onwards. Comprises large numbers of fail-safe electro-mechanical relays, hard wired together.

Computer Based Interlocking (CBI)

Solid State Interlocking (SSI) is the UK's original CBI, developed by BR in the 1980s. Safety by parallel processing with 2-out-of-3 voting. Controls trackside equipment via serial data to link to a Trackside Functional Module (TFM) which interfaces with points, signals, etc. Configured in a bespoke language which is then compiled.

Automatic Warning System (AWS)

A permanent magnet and an electromagnet of opposite poles are mounted between the rails, about 200m in rear of a signal. The electromagnet is only energised when the signal is green. If the train detects both, a bell is sounded to indicate green; if the train detects only one, a horn sounds, and if the driver fails to acknowledge within 2 seconds, the emergency brakes are applied.

Train Protection and Warning System (TPWS)

Antenna pairs are mounted between the rails, one pair just past the signal (the train stop sensor), and one or more pairs some distance in rear of the signal (the overspeed sensor(s)). All loops are energised only when the signal is red. When the train passes an antenna pair, the first antenna arms a timing circuit on the train; if the train passes the second antenna before the timer expires, the emergency brakes are applied. The train stop sensor has both antennas very close together, and will trip the train at any speed; the overspeed sensor has its antennas further apart, so as to only trip when the train is going too fast to stop at the signal.