THE SIGNAL BOX

BLOCK SYSTEM

BLOCK SYSTEM

2. Keeping the trains apart — Regulation 4

On this page:
[ Block instruments | Light relief | Method of working | Clearing point | Tail lamp | Train register book | In practice | Short sections | Special instructions ]


Block Instruments

Given that a each block section involves two signalmen, it is necessary to have a strict discipline in the way trains are admitted to the section to avoid collisions. This is achieved with Block Instruments.

Two block instruments

Illustrated on the right are a pair of block instruments that would control a double line piece of railway between two signal boxes. At the top of each instrument are two electrically controlled needles (D&E), capable of pointing at Line Blocked (Line Closed or Normal on some instruments), Line Clear and Train on Line. Beneath is a commutator handle (C) that allows the signalman to choose the position of the lower of the two needles. At the bottom is a bell tapper (A) - like a morse key - and a bell (B). The bell tapper rings the bell at the next box. The commutator controls the lower needle in this instrument, and the upper needle at the box at the other end of the section. Some older instruments do not have all of the equipment built into one unit; it is not unusual to find each needle, and the bell, housed in separate units on the block shelf.

Different types of instrument exist on some lines, such as Lock and Block, or two-position instruments. Whilst the basic principle of absolute block remains, the method of working the instrument is different, and will be detailed in future pages of this section.

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Light Relief

It all sounds very complicated, so ponder this.

The instrument shows Line Blocked when the line is not blocked, but shows Train on Line when the line is blocked. If the Line is clear, it shows Line Blocked but if it shows Line Clear, there is a train coming. If it says Line Closed, the line isn't closed, because if it was closed the block instrument would be taken away. It will show Train on Line if the train is on the line, but will also show Train on Line if the train is off the line. So, if the block instrument shows Normal, things probably are very normal indeed.

Now we've had some light relief, let's see how they are actually worked.

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Method of working

The block instruments are worked in the following manner. For a train to run from Box A to Box B, the method is as follows. When there is no Train in section, the block needles hang in the centre position - Line Blocked. The signalman at Box A can see there is no train in the block section ahead by checking the top needle on his block instrument.

He then sends one beat (the Call attention signal) on the bell tapper to Box B, which that signalman acknowledges. The signalman then "offers" the train to Box B by sending the appropriate Is line clear? bell signal. The signalman at Box B, provided he is satisfied that the section is clear, and the line is clear for a quarter of a mile beyond his home signal (his clearing point) and all points within that quarter of a mile are set correctly, acknowledges the bell signal and turns his commutator, which in turn turns his lower needle and the upper needle at Box A. to Line Clear. The signalman at Box A may now clear his signals.

As the train passes Box A, the signalman there sends Train entering section (2 beats) on the bell, which the signalman at B acknowledges and also turns his commutator, causing his lower needle, and the upper needle at Box A, to change to Train on Line.

When the train has passed Box B, the signalman there replaces his signals to on, and checks the train has a tail lamp. Once the train has passed his clearing point, he sends Call attention on the bell to Box A, and when answered, sends the Train out of section bell signal (2-1) which is also acknowledged. He then turns the commutator, and his lower needle back to Line Blocked. This also shows on the upper needle at Box A.

The times that all bell signals are exchanged is recorded in the Train Register book.

It will therefore be seen that both signalman have a permanent indication of the state of the block section, but the signalman at the box in advance (in this instance Box B) is the person who makes the ultimate decision as to whether the signalman at A can admit a train into the section. Indeed, if he has any reason to not wish the train to enter the section (for instance, if the line is not clear to his clearing point), he simply refuses the train by not acknowledging the Is line clear?signal and leaving the commutator and needles at Line Blocked.

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Clearing Point

The clearing point is a quarter of a mile ahead of the first stop signal controlled by a box, and is a safety zone - an area that must not be occupied or fouled by another train. It allows for the approaching train being unable to stop at the signal, perhaps through slippery rails or weak brakes.

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Tail Lamp

A tail lamp is a red lamp provided on the rear of all trains, and is a fundamental part of the absolute block system. It is the only proof the signalman has that the whole train has left the section.

If the thought of part of a train being left in the section seems unlikely, you need to think again. Consider the fact that in 1960, many freight trains had no automatic brakes other than those on the locomotive so a driver could be unaware that his train had become divided.

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Train Register Book

The Train Register is a large book, ruled out in columns, for the recording of all bell signals exchanged and also incidents. A signalman can tell, at a glance, the state of the block section by the number of columns that have entries in them.

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In practice

We have just looked at the operation of the instruments for one block section, but a typical signal box has two instruments and four block sections to contend with.

Layout at Signal Box "B"

Returning to our imaginary Box B, the signalling of an Up train would be as follows:

Bert, the signalman at Box B, would be disturbed from reading his newspaper by a the sound of a ding on the bell from his mate Arthur at Box A. He puts the paper down, and moves smartly to his block instruments, answering the Call attention back to Arthur. Arthur then offers the train to Bert. Provided the section is clear (which is confirmed by the position of the block needle and entries in the Train Register), and provided the clearing point is not fouled, Bert accepts the train by acknowledging the bell signal and placing his commutator and needle to Line Clear. After making a record of this in his Train Register he sits down and resumes reading his paper.

A few minutes later the bell rings again, this time with two beats - the Train entering section signal. Bert leaps to his feet and acknowledges this with two rings, and turns the commutator and needle to Train on Line.

Bert now needs to turn his attention to clearing his signals, because otherwise the train will get delayed, and he could end up on the carpet in front of the guv'nor if it does. But, before he clears his signals, he must have the train accepted by Charlie at Box C.

So he calls attention on the bell to Box C, and when this is acknowledged, he offers the train by sending the Is line clear? bell signal. Charlie accepts the train by acknowledging that bell signal, and placing the upper needle to Line Clear. Bert now knows it is safe to clear his signals, and does so.

A few minutes later, the train approaches the box. Bert sends two beats on the bell to Charlie at Box C, which is acknowledged, and the top needle changes to Train on Line. He watches the train pass, placing his signals to on behind it. Is the tail lamp present? Yes. When the train has passed the clearing point, he calls Arthur's attention on the bell and when answered, he sends Train out of section (which will be acknowledged) and places the commutator and lower needle back to Line Blocked.

Note - on the Western Region, the call attention signal is not used when sending Train out of section, nor is that bell signal acknowledged.

Job done? Not quite - after a few minutes the bell from Box C rings, and when he has responded, Charlie sends the Train out of section signal, and acknowledges it, and Bert notices the upper needle return to Line Blocked.

And don't forget Bert has made a note of the time each bell signal was received or sent in the Train Register while all this has been going on. It is also conceivable that a Down train may need to be similarly dealt with while all this is going on.

And now Bert can get back to the newspaper . . .

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Short sections

It will be appreciated that where signal boxes are very close together (at some locations they are less than ¼ mile apart) there is insufficient time with this method to clear the signals to avoid delays to approaching trains. In our example, Box C is quite close - Box B's distant signal is mounted on the same post as Box C's Starting signal, and Bert would delay Down trains if he waited for Train entering section before signalling the train to Box A.

For this reason special authority is given at certain signal boxes to offer trains immediately they have received them.

At other locations use of the Train approaching bell signal (1-2-1) is authorised to allow signals to be cleared in sufficient time.. How it is applied is defined locally, but generally is is sent to the box in advance when Train Entering Section is received from the box in rear. The allows the signalman at the box in advance time to offer the train to the next box and clear his signals.

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Special instructions

There is actually no such thing in reality as a "typical" signal box like our Box B. There are always individual factors that require minor adjustments to the basic rules to allow efficient operation. Every signal box has a "Special Instructions" card (often referred to as "Footnotes" on the Western region) which details any necessary variations.