I've always thought the Fletcher Double Needle Combined Block Signalling Instrument produced by Fletcher for the London & North Western Railway (illustrated above) an elegant and practical solution to the problem of the safe handling of trains using the block system.
Background: Railways were one of the first users of the electric telegraph to allow communication over long distances. The Single Needle Telegraph was based around a Polarised Galvanometer where an electric current flowing through a coil under the influence of a permanent magnet caused the movement of an armature carrying a pointer. Current in one direction deflected the pointer to the left, current in the opposite direction produced a deflection to the right. The current was produced by two batteries, connected to the galvanometer by pressing one of two keys.
A complete system required an instrument similar to the photograph above at each of two stations. The two instruments were interconnected by a single telegraph wire - the earth could serve as a return to complete the circuit. Batteries at each station allowed the two black keys at the base of the instrument to deflect the needles of both the local and remote instruments left or right, as desired. By rapidly generating a series of left and right swings, each letter of the alphabet could be represented, using the code printed on the dial visible in the picture above. Transmitting successive letters allowed words and complete messages to be generated. The telegraph quickly became a vital part of the commercial management of railway systems. It also paved the way for a system of train control on double lines so succesful that it remains in use today.
Requirements of the Absolute Block System: The Block System divides each line into sections extending from one signalman to the next. Only one train is allowed in each 'block section' at a time so as to avoid collisions. The block signalling equipment must allow adjacent signalmen to communicate so that the signalman in advance can, when requested, give the necessary permission to the signalman in the rear for each train to approach. This is done by the exchange of simple coded messages using single-stroke electric bells. In addition, there should be a continuous indication to both signalmen as to the current state of the section, occupied or clear. This is done using galvanometers as described above. Some systems used a non-polarised galvanometer with just two positions (Line Clear/Train on Line). The advantage of using the polarised galvanometer was that three distinct conditions could be indicated (Line Closed/Line Clear/Train on Line).
Use of Block Indicator/Bell/Block Indicator: Spagnoletti on the Great Western Railway developed successful equipment based on these principles, based on three separate wooden-cased units - a single-stroke bell, a block indicator with two keys and a block indicator without keys. A double track line between two signal boxes required a bell (used for trains in both directions), an indicator with keys (used for trains approaching) and an indicator without keys (used for trains departing) at each box. The keys in a telegraph system were pressed and released quickly but in the block indicator, a peg was provided which could be inserted so as to hold one key pressed for 'Line Clear', the other key pressed for 'Train on Line' or be removed for 'Line Closed'. The signalman in advance controlled the pegging of the keys and the signalman in the rear observed the block indicator without keys. From these origins, the two types of block indicators were called 'pegging' or 'non-pegging'.
Many railways continued to use the separate Block Indicator/Bell/Block Indicator arrangement. The picture below shows three demonstration Midland Railway units on the Battlefield Line arranged Block Indicator/Bell/Block Indicator. In this case, both indicators are 'pegging' instruments (although the two keys with a peg have developed into a 3-position rotary switch called a 'commutator'). In a real double-track situation, one block instrument would be 'non-pegging'.
Development of the Combined Instrument: However, in a large box, this Block Indicator/Bell/Block Indicator arrangement used up a lot of space on the block shelf. Fletcher produced for the L&NWR the 'Combined' instrument illustrated at the top of this post where the Bell, the Non-pegging Block Indicator and the Pegging Block Indicator (with its associated Commutator) were grouped into a well-proportioned unit. The commutator was arranged as a double-pole reversing switch so that a single battery could be used to produce the opposite polarities required for 'Line Clear' and 'Train on Line'. The design incorporated a number of features making the unit reliable and secure. More pictures.
Variations: The basic design appeared in a number of variations. One of the more common was the 'Permissive' instrument. On a permissive line, more than one train can be allowed into a block section, subject to certain safeguards. This was often done on Goods Lines. The permissive 'pegging' instrument incorporated a mechanical reminder of the number of trains currently in the section. Pressing the spring-loaded plunger on the right hand side of the unit would allow the commutator to be turned clockwise (to add a train) or anti-clockwise (as a train cleared the section) and the glazed window above the commutator would indicate the appropriate number. The 'More pictures' link above shows a permissive instrument formerly in used at Watery Lane signalbox.
As well as the Double Needle ('DN') pattern for double track, there were Single Needle patterns for use in appropriate situations. The 'Third Line' from Sedgeley Junction to Dudley East was such a situation.
The most unusual situation I came across was the Up & Down Goods Line between Deepfields and Spring Vale. Here, a single line carried traffic in both directions. Special 'pegging' instruments with separate 'Up' and 'Down' block indicators were provided at each signal box, but electrical interlocks ensured that only one box could give 'Line Clear' at a time. I made some partial notes back in 1964 but didn't get an opportunity to study the arrangement in detail. Sorry!