Railway Signals are used to control trains. There are different types of signaling methods. Main methods used in Sri Lanka are Electronic signals & Semaphore signals. Here we going to discuss about electronic signalling system of Sri Lanka
Electronic Signaling System
Electronic Signals are of two kinds as Automatic signals & Controlled signals. Automatic signals can be again divided into 2 sub categories as automatic blocking signals & automatic approach signals while controlled signals are divided as high controlled & dwarf controlled signals.
How to Identify Automatic & Controlled Signals?
These two types of signals can be mainly identified by their number plate. Automatic signal posts has a circular number plate while Controlled signal posts have a square shaped number plate.
These signals could contain one, two or three units on a signal post. These are approach lit signals. That means these signals only lighting when a train approaches. This technique is used to save electricity. These signals operate themselves according to the condition of the ahead track section. Following is an illustration of how automatic signals work.
To ease the controlling, railway track is divided to sections. Only one train can occupy a section. In the diagram above Train 2 is on Section 3. Therefore the signal at the beginning of that section shows red colour or danger. That means a train in section 2 cannot enter section 3. Post at the entrance to the section 2 shows yellow colour or caution. That means the next section is free but the section beyond it is occupied. This is used because a train cannot stop at once. As in the diagram section ahead the Train 1 is clear.
In automatic signaling, the system checks if these sections are occupied & then light up the lights accordingly. There are 2 main automatic signal types.
- Automatic block signals
- Automatic approach signals
Automatic Block Signals
Red: Danger. Train cannot go beyond the signal. Train on the next section.
Amber: Caution. Train on the section after the next. Prepare to stop at the next signal.
Green: Clear. Line ahead is clear for at least 2 sections.
Automatic Approach Signals
The meaning shows that a controlled section is ahead & the line will be taken at a controlled speed. These are also known as distant signals or outer home signals. Normally the line divides into 2 after this signal & the upper unit represents the main line & the lower unit represents the loop line. The meanings of this signal is as follows.
Red: Danger. Section ahead is occupied ahead & stop at the signal
Amber upon Amber: Caution. Ready to stop at the controlled signal
Green upon Amber: Line clear. Going to the main line
Amber upon Green: Line Clear. Going to the loop line
The controlled signals are controlled by a main controlling office. These signals are normally shows the red colour & these signals light up only when a train arrives to save power. These signals can be identified by the square number plate which is fixed to the signal post & the posts are painted in red & white stripes. There are 2 main types of controlled signals.
- High controlled signals
- Dwarf controlled signals
High Controlled Signals
These signals are situated just before a yard. Normally these signals have 2, 3 or 4 units. Out of these units the upper unit represents the main line & the second unit represents the immediate loop line. These are the normal arrangement of a high controlled signal but some signal posts have a third & fourth unit. The third line has only an Amber light. These signals are at places where more than two turnouts are available. The fourth unit is designed to represent a number & these signals can be seen before arriving Colombo Fort & Maradana railway stations. This unit represents the platform number which the train is going to arrive. First unit has no special speed restriction & second unit has a speed restriction of a 48kmph while the third unit has a restriction of 16kmph. Following are the meanings of these signals. Colours are from the top.
Red, Red: Danger. Stop. The signal is against.
Amber, Red: Caution. Going along the main line.
Green, Red: Line clear. Going to the main line.
Red, Amber: Caution. Going to the loop line.
Red, Green: Line clear. Proceed to the loop line at the restricted speed.
Red, Red, Amber: Caution. More than one turnouts.
Dwarf Controlled Signals
Apart from above described signals there’s another signal type called Call-on signals. High controlled signal posts cannot be used in a yard to give signals to a shunting locomotive because of the difficulties in seeing & the presence of the carriages on the same track. Call-on signals are used to overcome these problems. These signals can be seen in a yard where shunting takes place & it’s situated above the base of high mounted signals. These signals have 3 white lamps & its aspects are as follows.