Aspect Sequence Chart Railway Signalling

Definitions:

Aspect – any valid visual indication of a signal as displayed to the driver

Aspect sequence – The order in which the aspects of successive signals are displayed to the driver

Signal spacing – The distance between the signal or lineside sign displaying cautionary aspect and the signal or lineside sign at which the train is required to stop.

STANDARD THREE ASPECT SEQUENCES

GREEN – YELLOW – RED SIGNAL

STANDARD THREE ASPECT SEQUENCES

* A driver, running under authority of a green aspect in a three-aspect section of line, shall be warned of a stop aspect by the standard three aspect sequence. the minimum signal spacing distances between the yellow and red aspects for various speeds are set out in GK/RT0034.

* Where distances between successive stop signal is more than the maximum permitted by GK/RT/0034, separate distant signals shall be provided.

* Where the permissible speed of trains is greater than 100mph, four aspect signalling shall be considered.

GREEN – DOUBLE YELLOW – SINGLE YELLOW – RED

STANDARD FOUR ASPECT SEQUENCE

 

MODIFIED THREE ASPECT SEQUENCE

Where it is necessary to position signals such that the required spacing between a YELLOW and a RED aspect in the standard three aspect sequence cannot be provided, sufficient warning of the stop aspect shall be provided by means of one of the following:

1. An isolated standard four aspect sequence (preferred method)
2. Approach control from red of the previous signal to the signal with sub-standard spacing
3. The imposition of a speed restriction.

MODIFIED THREE ASPECT SEQUENCE

1. ISOLATED FOUR ASPECT SEQUENCE (PREFERED METHOD)

 

2) Approach control from red of the previous signal to signal with sub-standard spacing

 

MODIFIED FOUR ASPECT SEQUENCE

STANDARD FOUR ASPECT SEQUENCES

A driver, running under authority of a green aspect on a four aspect section of line, shall be warned of a stop aspect by the standard four aspect sequence.

The minimum signal spacing distances between the double yellow and red aspects for various speeds are set out in GK/RT0034.

Green – Double Yellow – Single Yellow – Red

Where it is necessary to position signals such that the required spacing cannot be provided between a Double Yellow and a Red aspect in the standard four aspect sequence, Double Yellow aspects shall be additionally displayed at one or more preceding signals such that minimum signal spacing distance is provided between the outermost Double Yellow aspect and the Red aspect.

• Green – Double Yellow – Double Yellow – Single Yellow (*) – Red

* Train Protection System fitted as a minimum to this signal

• As an alternative to above – restrict the permissible speed

CHARLES WEIGHTMAN’S RECOMMENDATION

Where it is necessary to position signals such that the spacing between the single yellow and the red aspect does not meet the requirement for the 4 aspect sequence (one third of the applicable signal spacing distance), then one of the following modified aspect sequence shall be adopted.

Green – Double Yellow – Single Yellow – Red/controlled to Single Yellow – Red

Green – Double Yellow – Single Yellow – Single Yellow(*) – Red (*) TPWS

 

ASPECT SEQUENCE TRANSITIONS

Segregation of three and four aspect sequence:

* The number of transitions between three and four aspect signalling shall be kept to a minimum

* It is permissible for an isolated four aspect sequence to be displayed in an otherwise three aspect sequence

AT THE TRANSITION ONE OF THE FOLLOWING ARRANGEMENTS SHALL BE APPLIED

 

* THE FIRST FOUR ASPECT SIGNAL DISPLAYS A SINGLE YELLOW UPTO THE NEXT SIGNAL AT RED OR A DOUBLE YELLOW UP TO THE NEXT BUT ONE SIGNAL AT RED

 

* THE FIRST FOUR ASPECT SIGNAL SHOWS ONLY RED/DOUBLE YELLOW/GREEN AND ADDITIONAL DISTANT SIGNAL IS PROVIDED BETWEEN IT AND THE  NEXT STOP SIGNAL AHEAD

 

* THE FIRST FOUR ASPECT SIGNAL IS APPROACH CONTROLLED FROM RED TO YELLOW WHEN THE NEXT SIGNAL AHEAD IS AT RED

 

PROHIBITED ASPECT SEQUENCES

Successive signals shall not simultaneously display the following:

a.A red aspect immediately preceded by any colour light aspect other than a single yellow aspect

b.A single yellow aspect followed  by any aspect other than  red, as permitted in modified sequence