Midland Rotary Interlocking Block


Midland Rotary Interlocking Block

Probably the safest block instrument in the world

by John Hinson

Up to the turn of the century, the only block instrument in use on the Midland Railway was a conventional three-wire three position pegging instrument, similar to those found on many other company’s lines, many of which had probably been converted from the quantity of surplus telegraph instruments from the pre-block era.

There was nothing intrinsically unsafe about these instruments – they were actually superior to the one-wire, two position blocks found on many other company’s lines at this time.

What was ahead of them in terms of safety, though, was the Sykes Lock and Block system. Through a complex array of instruments, the Sykes equipment prevented one train proceeding beyond a signal until something physical indicated that the previous train had passed the next signal. This was achieved either by mechanical train-detecting treadles in the track, or by the action of clearing and replacing a signal lever.

Whilst the latter method of train detection may seem a little vague, it will be understood that if the Sykes principle is applied to many successive signals, it is impossible to “lose track” of a train.

The signalman would be faced with an array of polished and varnished cases – one for each signal lever – with a mechanical indicator showing “LOCKED” and “FREE”.

But what could be done if a wrong signal was pulled, or a failure occurred. Worse still, the equipment could not normally even cope with trains shunted into sidings, and in such cases it would not be possible to clear the protecting signal again for the next train.

This problem was overcome by the worst cop-out I have ever seen. Hanging on the block shelf in every signal box would be a Sykes Release Key which could be used to release the interlocking.


Langdon’s Block

I believe the Midland Railway saw and admired the principle of Sykes Lock and Block. I think they set about designing their own equivalent, avoiding all the pitfalls of Messrs. Sykes’ system.

Their first type of secure, interlocked block was know as the Langdon’s type after the Midland engineer that designed them. The Langdon’s block was really just an accessory-filled version of the traditional pegging block instrument.

Langdon’s block instrument
John Hinson,
Langdon’s block instrument with case removed
John Hinson,

But this type was soon overtaken.

Rotary Interlocking Block

The Midland went on to produce a dedicated instrument that:

  • looked and worked like a conventional three position instrument
  • was not released by a readily-accessible key.
  • did not require providing for every signal lever
  • catered for shunting and other unanticipated working.

The wooden case was basically similar to the ex-telegraph instrument three-position type, but it was deeper to accommodate the complex mechanism. In place of the pegging handle was a smaller handle which rotated through 360 degrees—hence becoming known as “Rotary” block. Behind the handle was a circular brass plate (often well polished) divided into three equal segments corresponding to the three needle positions for “Line Blocked”, “Line Clear” and “Train on Line”. The handle was normally only turned in a clockwise direction, and a ratchet prevented reverse operation except in special circumstances. Operation was as follows.

    1. When accepting a train, the signalman would rotate the handle clockwise from “Line Blocked” to “Line Clear”. The needle itself would only turn to correspond with the commutator if the Home Signal lever was in the Normal position and, if that signal was repeated, the arm was proved in the “On” position. This would release the starting signal at the box in rear to be pulled for one train only.
    2. The handle could not be restored anti-clockwise from here to “Line Blocked” unless both signalmen operated a co-operative “Line Clear Cancel” button on their instruments which would electrically release the ratchet.
    3. When the train entered the section, the signalman would rotate the handle clockwise to the “Train on Line” position, in which it was now firmly locked.
    4. When the train passed, it would operate a mercury controlled train detection treadle under the track near the signal. Provided the home signal was in the clear position at the time, the lock on the instrument would be released. However, if the signal was not replaced behind the train, the needle would remain at the “Train on Line” position.
      In later years, some treadles were replaced by sequential occupation and clearance of track circuits at the home signal.
    5. If it was necessary to release the instrument from “Train on Line” in any other circumstance, an emergency glass-protected plunger had to be operated.
    6. Blocking-back manoeuvres were catered for by a “Blocking Back Inside” position which could be reached by turning the handle anti-clockwise from “Line Blocked” anti-clockwise to a position just ahead of “Train on Line”, and thus not subjecting the instrument to the usual treadle release. However, instructions dictated that if the move was proceeding outside the home signal, the handle had to be turned to “Train on Line” through the normal sequence via “Line Clear” in order to fully lock the instrument.

Midland Railway Rotary Interlocking Block Instruments

Pegging Instrument (left), used by the signalman at the “accepting” end of the section, and non-pegging instrument (right), used by the “sending” signalman.

These early photographs show the additional feature originally provided to allow the block needle to be waggled to indicate the routing of the train. (see below)



Rotation Locking

The principle of applying interlocking between signals was achieved by means of “Rotation Locking” between the levers. This achieved exactly the same as the Sykes system but carried out by the lever frame’s own interlocking and without the complex and expensive instruments. Rotation locking was not generally provided where track circuits existed, and was often removed from when track circuiting was later added.

Rotation locking works by mechanically locking a signal lever after it has been operated (thus protecting the train that has obeyed the signal and moved forward) until the next signal lever has been pulled and replaced (when the train would have moved on past that signal).

At some locations, the rotation locking was applied to shunt signals, too. If a signalman pulled a wrong lever in error, it was often necessary to shunt the imaginary train (that the locking thought was there) around until it was deposited in a siding!


Routing Information

A further feature, provided on the original instruments in complex areas, was a legacy from single-needle telegraph days and also could be found on the conventional three-position blocks of the Midland Railway. On the non-pegging instrument, which was converted from a redundant telegraph instrument, the handle was retained and could be used to waggle the block needle to indicate to the adjacent signalman (in code) the destination of the train.

The pegging instruments had a small tapper key below the commutator handle to acknowledge messages.

These operations apparently used a much lesser voltage for the operation, to ensure that activity did not, for instance, activate the “Line Clear” release on a Stating signal. These were taken out of use in LMS days (probably through difficulty establishing suitable voltages when intermediate boxes switched out), and superseded by routing bell signals.

The tapper key was removed from all of the pegging instruments and the space panelled over, although the non-pegging instruments generally retained their now non-working handles to the end.


Short Sections and Long Segments

It seems to have been recognised that where busy boxes existed with short block sections, the train might pass over the release treadle before the signalman has turned the instrument to “Train on Line” and therefore not release the instrument.

I don’t know how long it took them to discover this, but it is an interesting point that the Company recognised that Rules and Regulations can sometimes be difficult to implement “by the book”. Perhaps it was the lineman’s order for 500 new glass seals that did it!

Anyway, a special variation of the instrument was provided for short block sections where the treadle would release the instrument whether at “Line Clear” or “Train on Line”. As the release worked on two-thirds of the full circle of the instrument, these became known as Long Segment instruments.

It’s really easy to remember, ha ha – short segments for long sections and long segments for short sections. Despite this risk of confusion, and the occasional need to replace an instrument, I know of no occasion where the wrong type was fitted. However, I have also seen two such instruments in ex-works condition wrongly labelled!



Use of Rotary Block instruments, which must have been quite expensive, was widespread on busy sections of line. For instance, every block section on the passenger lines between St. Pancras and St. Albans South was provided with these instruments.

Piecemeal provision was also carried out following accidents, for instance, after the famous Hawes Junction collision of Christmas Eve, 1910.


Later modification

The LMS made modifications to some instruments in the 1930s to prevent the instrument being turned to “Line Clear” if the distant signal was not correctly in the “on” position. This could be released by another glass-covered plunger when necessary. This release later became redundant when it was decreed that in the case of such a signal failure, mode of working should be in accordance with a block failure.


The test of time

The only time the instruments would become unsafe would be if the Train on Line release glass was missing. This became more commonplace in later years as with higher train speeds and consequent hammering of the track, treadles would frequently become defective or out of adjustment. Additionally, a diesel shunting engine, or a fast moving light main-line diesel engine would not always activate the treadle.

However, any secure system poses a challenge to a wily signalman, and the following cheating methods are known of:

  1. Wardrobe keys often fit the locks on the instrument cases
  2. Temporarily releasing specific contacts in the closing switch would give a second “Line Clear Release”. (In early days closing switches were locked and the key held by the Station Master).
  3. A poker or screwdriver could be poked through the hole later vacated by the LMS distant release plunger could be manipulated to release the Train on Line release.
  4. If the relay cabinet could be accessed, turning the track circuit relays over could be used to imitate the passage of a train over track circuits.

Use of these illegal methods was always, of course, to overcome failures rather than to achieve two trains in a section – and any signalman caught in the act of such fiddling would find his career abruptly terminated.

Nevertheless, the instruments stood the test of time and usually worked well. From a signalman’s point of view they were reliable and could be trusted.

So reliable they were, in fact, that a white diamond would be fitted to the post of home signals even if track circuits were not provided. The interlocking of the instrument was considered sufficient to exempt drivers from having to immediately remind the signalman of their presence under Rule 55.


The competition

Was there any other instrument in the running? Not really. The majority of railway companies settled for conventional three position blocks, with add-ons such as “Line Clear, One Pull”, Welwyn Control and Track Circuiting. The Southern Region’s “Closed Block” system incorporated some features of Rotary Interlocking Block like co-operative cancelling. A few areas laboured on with primitive one-wire, two position blocks and a few of these survived into the late eighties or early nineties.

The Metropolitan Railway standardised on Spagnoletti Lock & Block, which was similar to that used by the Great Western but with treadle releases to confirm the train had left the section. They were not as effective as the Midland’s Rotary block in that there was no “Line Clear Cancel” facility nor any rotation interlocking.

I understand there was a form of Rotary Interlocking block used between Watford No2 and Watford Tunnel North End on the former L&NWR main line. From descriptions from former signalman, these do not appear to be standard Midland instruments but I have no other details. It would be interesting to learn more about these.

The majority of Midland Rotary Interlocking Blocks were installed on busy lines that have subsequently been resignalled, so numbers fell quickly. The last-surviving examples were at odd locations on the Settle & Carlisle line, but these were finally removed on 11th September 2002 (owing to a lack of spares for them) and replaced with less-secure standard blocks.

Fond memories – a crowded block shelf with four sets of Rotary Interlocking block instruments at Napsbury:

Napsbury SB
John Hinson, c1975


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