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There were many schemes in the 20th Century (Crewe, Hull, LSWR in the Southampton area, Southern Railway after 1923, East coast mainline around Thirsk etc) where some form of power signalling was in use. They included electrical solenoids, electro-pnumatic, electric motor etc. I warmly recommend "Two Centuries of Signalling History" by Geoffery Kitchenside and Alan Williams, published by OPC in 1998 for a quite detailed and well-illustrated tour of signalling developments.The electrical control of signals and points means that the wires and rods are replaced by cables. The cables were originally strung on hooks mounted on posts or attached to the walls of cuttings and tunnels but by the 1960s they were usually laid in the white rectangular trunking alongside the track (see Fig___). Where the cables pass under the rails it is usual to run them through a length of tubing, from the 1980s this was usualy about two inches in diameter and coloured orange.
Regarding your comments on the spacing of signal boxes. There were two major factors -
(a) Limits by the Board of Trade on the distance over which manually operated points could be worked, finally set at 350yards
(b) The complexity or lack of junctions in the area and the quantity of traffic. The former would require more boxes where junctions were close together; the latter might, for instance at a large station, require two or more boxes to operate the signals just to divide the work-load up to minimise potential delays.
Another factor affecting the spacing of signal boxes was the introduction of the 'Intermediate Block Signals' system. This used electrically operated semaphore or colour-light signals together with track circuits worked from a distant signal box. This system was used to either divide up long sections of track so that more traffic could be handled, or to replace 'break-block' signal boxes put in with the same intention but possibly difficult to man reliablely.
The Railway Clauses Act of 1845 introduced the law that all railway property must be adequately fenced off from the public. Several early railways had already started fencing off their property to avoid accidents and the Liverpool and Manchester railway even had gated level crossings on its line, each attended by a company 'policeman'. The gates were normally closed to road traffic and opened by the policeman as required. The practice of having the road normally open and the railway line blocked by the gates started in the latter part of the 19th century, when the electric telegraph was introduced enabling gate keepers to be advised of an approaching train. Level crossings were usually equipped with some dregree of street lighting, usually this was provided by the railway company although the lights themselves might be conventional street lights mounted outside the gates on the pavements. The Ratio 'platform gas lights' serve well enough as the kind of lighting often provided. It is perhaps worth noting that at some crossings, at least in country areas where there was no street lighting as such, the side facing up the road was fitted with a red glass, providing a warning to road users, the other three sides being plain glass to illuminate the crossing itself. This was not universal, in built up areas all the faces of the lanterns were plain glass, such as the post mounted lights at Hale illustrated below
On quiet lines the gates were operated by hand, a man would walk one gate shut then the other, in these situations there were usually only two gates used. The gates on busy lines were operated by a mechanical linkage operated from an adjacent signal box. There was a large wheel, mounted at the end of the box closest to the crossing (see Fig___), this meant the signal man did not need to leave his box to operate the crossing. The upper drawing in the sketch below shows the asymetric set of gates at Hale on the CLC as they were in about 1910, when I remember them they had been painted white and a red 'bullseye' added to the longer gate (this ended up in the centre of the crossing when the gates were closed, see Appendix 5 for a sketch). The lower drawing shows a 'typical' set of symetrical gates. The gate to the side, allowing pedestrians to cross the line, was linked to the main gates on the roadway, those I remember locked shut when the main gates were closed. As I remember it women with prams and the like had to use the roadway to avoid this gate.
Fig___ Level Crossing Gates
If a signal box was not conveniently sited a small house was often provided for a 'crossing keeper' who's job it was to open and close the gates, these gates would be operated by hand rather than by mechanical linkage. On quiet rural lines the train crew often had to operate the gates when they reached them and this system was still in use on at least one rural line in the North East as recently as the late 1980's.
The road surface on the crossing was built-up to rail height so that vehicles could cross the tracks, where the gates were operated by a wheel in the signal box the surface of the crossing was made up of timber to allow access to the 'works' underneath. At crossings where the gates were operated by hand the surface of the road might be standard macadam or later tarmac. It was common practice to add timber baulks just inside and outside the rails on these latter crossings to reduce the sideways pressure of road-vehicle wheels on the rail and ease repairs to the track.
When the Light Railways Act was passed in 1895 the newly designated light railways were allowed to do away with gates at their level crossings. The Weston, Clevedon and Portishead Railway became a Light Railway in the late 1890's and they actually used the level crossings as halts. The company had bought some American looking coaches (originally built for export to South America), which had open platforms with steps at each end. These were operated in pairs and the train was stopped with the centre pair of platforms on the level crossing for people to get on and off.
One problem with un-gated crossings was that cattle being herded along the road could wander off up the railway track. To avoid this it was standard practice to add a set of triangular wooden beams called a 'cattle grid' across the track. The cattle could not walk over these grids (humans often have trouble with them as well). Where a farm access road crossed the line and the lay of the land allowed it was normal to arrange a bridge or a short tunnel (called a 'cattle creep') to allow the animals to be moved from one side to the other. Where the crossing was on the flat there would usually be gates in the fence to allow cattle to be herded across the line (usually referred to as an 'occupation crossing'), but these would be arranged to open away from the track so if left open they would not be a hazard.
At un-gated crossings there would normally be a sign to warn road users and the railway line would have 'whistle' signs on the approaches to the crossings. The sketch below shows such a crossing, as shown this would serve for layouts from the 1920s to about the 1970s. After that date modern signage would be expected and warning traffic lights would be present (usually with the emergency telephone on the post supporting the lights).
Fig___ Level Crossing
As traffic increased in the 1930's accidents at un-gated crossings became an increasing concern, one option (used on the Weston, Clevedon & Portishead Light Railway among others) was to fit traffic lights operated by a pressure switch on the railway track.
All level crossings, gated or open, would have a standard cast metal warning sign set on a post on the approach road. From the 1904 both gated and ungated types had a distictive cast metal warning sign with an open centre red triangle on top (as shown below bottom left) set some distance (typically 100yds or so) from the crossing. The sign changed in the mid 1960s (along with all other road signs), although in more rural locations the old signs lasted a for a very long time. With the introduction of the lifting barrier type, all of which are considered 'gated' even if they have only half barriers, an additional notice was added to the sign as shown in the sketch below.
Fig___ Level Crossing Warning Signs
The first lifting barrier type level crossing in the UK (based on a French design) was set up near Uttoxeter on the Churnet Valley line in 1961. As these lifting barriers to not block the railway line these crossings require cattle grids to either side to prevent stray animals walking up the line.
The lifting barriers can be operated by a signal box (located some distance away but watching the crossing via CCTV camera) or they can be automatic. The automatic lifting barrier crossings use half-barriers which only block the left hand lane of the approaching road, leaving the exit from the crossing clear. The lifting barrier crossings are equipped with a pair of red light (flashing alternately) above an abmer warning light. They usually have a siren of some form that operates when the amber light is showing, prior to the barriers lowering. At my local crossing the lifting barrier was fitted with a bell, whoich was changed for a siren following complaints that people in the local pubs thought the bell indicated 'last orders' (this was when pub opening times were restricted by law and a bell was rung to advise people they had five minutes to order and drink).
The automatic half-barrier type is fully automatic with the gate motors controlled from the track circuits supplemented by wheel operated treadles to give more accurate timing. A sign is usually placed on the pole supporting the flashing lights, warning people that if the barriers stay down then another train is comming (to discourage people from 'weaving through' the barriers). Also on this pole is an emergency telephone to contact the signal box for the section in an emergency. There are usually double white lines down the centre of the road for some distance either side of the crossing, indicating 'no overtaking' to prevent people doing so and finding themselves on the crossing. In the example shown below the detail was taken from a photogaph, the white lines had evidently been recently repainted as they do not usually extend across the rubber decking of the crossing itself (where road marking are painted on to these it is not unusual to see the blocks put back in a different order, leaving elements of the markings in some rather odd places).
Fig___ Typical automatic 'half barrier' level crossing
Note that, up to the 1950's, the track at a level crossing with swinging gates usually had a timber inflill between the rails so that the 'works' underneath could be serviced. Where lifting barriers were employed the timber infill was replaced by pre-cast concrete slabs set into steel frames and by the late 1990's this had been replaced by pre-formed rubber blocks with a tread pattern moulded onto the top surface.
Fig___ Photo of cattle grid and concrete in-fill
There has been considerable debate regarding the delay required between the barriers coming down and the train arriving at the crossing especially where 'half barriers' are used. If the delay is too long there is a danger that motorists will try to drive round the barriers, too short a delay and slow moving vehicles might not get clear of the crossing in time. In 1968 a 120 ton transformer being moved by road was hit by a train at one of the new automatic half barrier level crossings. The tractor units pushing and pulling the loaded road trailer could only travel at a couple of miles an hour and the transformer was already on the crossing when the barriers came down. The crew of the lorry, and their police escort, failed to use the track side telephone to check with the railway staff in spite of a large sign instructing them to do so when transporting an abnormal load.
There is a story, probably apocryphal, that in certain parts of the country they had to change the wording on the warning signs for motorists. The story goes that the original wording was 'Wait while lights are flashing', but British Railways forgot that in some dialects the word 'while' means 'until' and a farmer complained to his local station that he had waited nearly an hour for the lights to start flashing then had to pull off the crossing very fast as the barrier started to descend.
On narrow roads a single gate was often used, these could be operated by a mechanism in an adjacent signal box, by a crossing keeper walking each gate in turn or (in remote areas) by the train crew. The example shown below was close by a small station on a double tracked line and was (I elieve) hand operated as there was no signal box close by. Where the road was narrow and the railway line was single track the gate itself could be quite small, but that is still twice as wide as a typical farm gate. Single gate crossings were used on minor roads, more so in the country than the town. These single gate crossings could be replaced by a single lifting barrier, either a 'half barrier' across one lane of the road or (as shown here) a full barrier that extends right across the road.
Fig___ Single gate and single barrier crossing
At busy crossings and in built up areas it is normal to install two pairs of lifting barriers which form a complete barrier across the road. As these block the road and might trap an unwary motorist or pedestrian they are usually remotely operated from a signal box or control centre. These are usually some distance away and at my local station there is a tall post mounted next to the track with a pair of TV cameras to allow the signalman to see the crossing. When open there is no barrier across the railway so again cattle grids are required to either side of the crossing. The example shown below is the current double-barrier set at Hale, south of Manchester. Sketches showing the earler swinging gates used at this location in 1910 and in the 1960s can be seen in Appendix 5 - Hale Station.
Fig___ Double barrier crossing
When the barriers reach across the whole road they normally have a 'skirt' of metal rods suspended below them to stop children and dogs running under the barrier getting onto the line. When the barriers are raised these fold back under gravity to lie close to the barrier and are difficult to see.
Fig___ Drop-down 'skirt' on barrier crossing
Replacing the older gated crossings has proved a long slow business and even at the end of the 1980's there were still a lot of the old manually operated swinging gate crossings in use. In places where road and/or rail traffic was high British Rail and local corporations and councils have worked together building road over-bridges to eliminate the level crossing entirely.
By the mid 1990's there were over four hundred half-barrier crossings in use and about three hundred full-barrier remote controlled crossings. In more remote locations no barriers are used but a set of warning lights is provided. At locations where crossing may be occupied for some time (for example cattle may be herded across the line) a telephone is provided, linked to the local signal box or control centre, so people can check the line will remain clear. There are nearly a thousand of these un-gated crossings equipped with lights and a telephone but there are another four and a half thousand un-gated crossings on the system with only a warning sign. Most of these are in built up industrial areas, docks and the like.
The 1845 Act required all railway company property to be safely fenced off but this did not apply to private sidings. In dock areas and large industrial estates there were often railway lines crossing roads with no gates, lights or other mechanical aids. There would be a sign (usually BEWARE TRAINS CROSSING) and the railway or industrial staff would provide a man with a red flag to stop the traffic when a train was crossing. This man would ride on the front steps of the loco which was usually travelling slowly in these areas.
Similarly where a siding from the railway crossed a road in to a factory the normal practice was to have a gate across the railway access and another across the entrance to the factory but the track between was often unprotected. My local town gas works in Altrincham was some distance from the railway station but was connected by a line laid in the middle of the road from the railway coal yard to the works. There was a gate across the entrance to the railway yard with another at the gas works and the rakes of coal, coke and tar wagons were hauled up the road by gas company locomotives. They had a small steam railway saddle tank engine and an unreliable diesel shunting loco but the job was often done by a steam road lorry also owned by the works.
One oddity on the Altrincham line is at Deansgate Junction signal box (which controls the diverging line to Skelton Junction and now operates the signals on the Chester line through Altrincham and Hale), in the 1930s when the line was electrified there was quite a lot of industry in the area, hence a number of large lorries with even larger loads were expected to use the level crossing. To prevent people hitting the wires the simple expedient adopted was to run a cable across the road on each side from which were suspended a number of cow bells. Non of the other level corssings on the line have had these (at least since the later 1960s) and to date I have traced no formal explanation or other location equipped with these bells.
Fig___ Cow bells still in place at a barrier crossing in 2007