THE SIGNAL BOX

BLOCK SYSTEM

BLOCK SYSTEM

3. Fouling the clearing point

On this page:
[ Refusing trains | Stationary obstructions | Setting back | Short sections | Train already in section | Swapping around | Inside or outside | Exemptions ]


Refusing trains

Supposing Bert, at Box B, wanted to cross a light engine from the Down Line to the Up Line at his box through points 9. He would be fouling his clearing point while doing so, because he would no longer have a quarter of a mile ahead of his home signal clear.

If Arthur, at Box A, was to offer a train while he was carrying out this manoeuvre, Bert should not accept it. He refuses the train by not acknowledging the Is line clear? bell signal, and leaving his indicator at Line Blocked. Arthur will repeat the Is line clear? signal at intervals to remind him.

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Stationary obstructions

However, if the above Light Engine is likely to come to a stand within the clearing point, he must take further precautions by "Blocking Back" to Arthur. He does this by, after calling Arthur's attention, sending the Blocking back inside home signal bell signal (2-4), which will be acknowledged, and placing his commutator and lower needle to Train On Line.

When the clearing point is again clear, he sends the Obstruction removed signal (which is identical to the Train out of section bell signal) and replaces to commutator and needle to Line Blocked.

Arthur will not offer him any trains during the period Bert has "Blocked Back", unless Regulation 5 is authorised, which will be discussed in Chapter 5.

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Setting back into the block section

The need to obstruct the block section itself from the advance end of the section is less common, but there was occasionally a need.

Layout at Signal Box "B"

A Down train might arrive and terminate in the Down platform. It would then be shunted across the crossover points (9) onto the up line, but would need to set back into the block section to start its return journey from the Up platform.

Before even shunting the train across the crossover points, Bert needs to send Blocking back outside home signal code (3-3) to Arthur. The train can then be crossed to the Up line and shunted back into the platform. Later, after the train has gone, and the block section and the clearing point are again clear, Bert can send Obstruction Removed.

In carrying out this manoeuvre, the train also occupies the section towards Box C. The method of signalling a train Shunting into forward section is discussed later, in Chapter 8.

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Short Sections

A complication with Blocking back outside home signal can arise where the block sections are short, because the set back manoeuvre could foul the clearing point at the box in rear. So, if Arthur's first home signal is less than a half a mile from Bert's home signal, Arthur must not accept a train from the box in rear of him unless he knows the movement Bert is making has come to a stand.

Why half a mile? Because with two potential movements towards each other, a ¼ mile "clearing point" is needed for both trains.

Bert will, in these circumstances, send Train or vehicles at a stand (3-3-4) as soon as he knows the movement has stopped and no further movement is intended.

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Train already in section

Should a train arrive within the home signal, but then need to set back outside the home signal, Bert at Box B would send the Blocking back outside home signal for train already in section bell signal (1-2-3) to Arthur, and not carry out the move until this signal was acknowledged.

Whilst the purpose of this signal is to cater for short sections (as described above), the description of the bell signal is complete rubbish, because the train must have left the section in order to set back into it!

This bell signal was only used by the Western Region.

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Swapping around

There may be occasions when Blocking back inside home signal needs to be changed to Blocking back outside home signal, or vice versa. In these circumstances, Bert and Arthur need to come to a clear understanding on the telephone as to what is happening, then Bert will send the new "Blocking back" signal on the bell to Arthur.

This is the only circumstance where the previous "Blocking back" signal is not finalised with sending Obstruction Removed.

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Inside or outside

The terms refer to whether the obstruction is within protection of the box's signals (inside) or in the block section (outside).

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Exemptions

Certain types of obstruction within the clearing point are permitted, as they are not deemed to be permanent obstructions. These include: