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This page describes the mechanical-era signalling equipment on the Callander and Oban Railway which, generally speaking, was typical of Caledonian Railway practice.
The line's original 'signal boxes' were really no more than lever frames on open stages. Some were subsequently covered so as to provide shelter for equipment and staff. Most signal boxes were of brick or timber construction (the 1902 signal box at Loch Awe was made of concrete). All the signal boxes on the Ballachulish branch were of timber construction. Signal boxes at remote locations were structurally combined with the signalman's house (the block bells were repeated inside the house). The last new signal box was the replacement Oban Goods Junction S.B., opened in 1929. It was brick built, to a standard LMS design.
The original Tyer's tablet instruments dispensed either round or octagonal tablets for alternate sections. In 1906 (and in 1915), the tablet instruments were a mixture of No.1 ("not shunting") and No.7 ("shunting") types (the Caledonian Railway referred to the latter as No.3). Within a few years, the No.1 instruments between Callander and Oban had all been replaced by No.7s, in some places accompanied by No.6 instruments issuing square tablets for long section working. The tablet instruments were often situated in the station buildings. From 1887, a banking staff was available for banking engines assisting goods trains up the steep gradient between Oban Goods Junction and Glencruitten Summit. This facility was withdrawn upon the opening of a signal box at the summit. A pair of auxiliary tablet instruments was provided at Killin Junction in 1935.
The Ballachulish branch originally had No.1 tablet instruments throughout; some were later replaced by No.7s.
The closure of the line east of Crianlarich in 1965 eliminated the only pair of key token instruments on the whole route at that time. They had only been in use for a few years. Subsequently, key token instruments replaced all the tablet instruments between Crianlarich and Oban. An intermediate key token instrument was provided at Crianlarich ground frame in 1967 in order that trains could "shut in" at the sidings. A "no signalman" key token instrument was provided at Oban in 1982, when the signal box was abolished.
The block instruments for the line's short double track sections were of the two-position Tyer's Caledonian type. Interlocked block instruments were used for bi-directional platform lines.
As was normal on the Caledonian Railway, most of the lever frames were of the Stevens Glasgow pattern. Remarkably however, Saxby & Farmer Duplex frames were installed at four new signal boxes opened in the mid-1890s: St. Bride's Crossing, Awe Crossing, Crianlarich East and Crianlarich West. The line's longest lever frame was the 64-lever Stevens frame in the 1904 Oban Station signal box. Frames were usually installed at the fronts of signal boxes (so that signalmen faced the track while working the levers) until early in the twentieth century when the normal practice was reversed to put them at the back of the box.
Evidence from old photographs seems to indicate the early use of wooden signal posts. However, steel lattice posts, topped by Stevens 'ball and spike' finials, would dominate the line for most of its life. Originally, the semaphore signals were all fitted with lower quadrant wooden arms. Some new semaphore signals provided in connection with the enlargement of Connel Ferry station in 1903 had a relatively uncommon pattern of spectacle plate with pear-shaped lenses, 60° apart. They were the last survivors of their type. Unusual arms, with spectacles to the left of the spindle and a circular balance weight to the right, were used in the Pass of Brander for the stone signals that carried arms for both directions; these allowed a single lamp to be used for both directions (one arm on each post being of the standard pattern). There were some unusual RSJ signal posts at Callander. Newer semaphores had tubular posts and steel upper quadrant arms. Surviving lower quadrant arms were gradually replaced by upper quadrant arms on the original posts. Latterly, some mechanical signals were electrically lit (the Pass of Brander stone signals still are). The Callander and Oban Railway never had any colour light signals (apart from one that controlled vehicular traffic at the south end of Connel Ferry Viaduct).
Oban featured examples of certain types of signals not seen anywhere else on the line. There was one 'skeleton' signal arm there. Oban was the only place to have subsidiary signals, of both shunt-ahead and calling-on types. The line's only route indicators were to be found on the signal gantry at Oban, comprising one stencil type and one theatre type.
At first, ordinary semaphore signals were provided for shunting purposes. From the late 1880s, dwarf shunting signals of the Stevens 'flap' type, fronted by rectangular targets, were being installed. These signals were usually ground mounted though they were in some cases elevated, for example when co-located with a main semaphore signal. The flap signals at Oban appear to have been lit by gas. Most of the elevated flap signals were replaced by miniature semaphore arms c.1920. Some dwarf semaphore shunting signals were installed during the LMS era at Connel Ferry. Later shunting signals were discs of the standard LMS pattern, the most recent of these being one provided at Dalmally in 1984.
Originally, all the crossing loops between Callander and Oban had weighted points at each end. These routed arriving trains into the left-hand loop and were trailed through by departing trains. They were not connected to any signal box lever but the blades were mechanically detected in the correct position when the distant signal lever was pulled. Facing points on Caledonian Railway lines were commonly provided with "Stevens' detectors" if facing moves were signalled over both the normal and reverse directions. Individual levers locked the points in the normal or reverse positions. Each operated a separate lock plunger working in conjunction with a bar that was common to both. They were gradually removed and replaced with conventional facing point locks and bars, worked from one lever. There was a 'split' lock bar at Dalmally. Economical facing point locks, operated by the same levers as the points themselves, were provided in the sidings at Crianlarich Junction. Since 1935, there was one set of motor-operated points at Killin Junction, which were too far from the signal box to be operated mechanically. The last surviving lock bars at Dalmally and Taynuilt were removed when track circuits were installed in the early 1980s.
The first place on the route to have track circuits installed was Killin Junction, in 1935. The resignalled layout at Callander East in 1958 also featured track circuits, which electrically locked the facing point lock levers and allowed the mechanical FPL bars there to be dispensed with. Similar work occurred next at Oban Station, followed in the early 1980s by Taynuilt and Dalmally. Upon the closure of Oban signal box in 1982, a single track circuit was installed in the Platform Line at Oban station. RETB swept away all the track circuits between Crianlarich (exclusive) and Oban.
A treadle was provided at Callander East to operate an annunciator for Down direction trains from Drumvaich.
The typical ground frame for working an outlying siding connection comprised a small two-lever dwarf frame, released by the section tablet. One lever worked the facing point lock (by the first half of the stroke) and the siding exit signal (by the second half of the stroke). The other lever worked the points. The instructions for working the tablet frame and ground frame can be viewed here.
There were electrically released ground frames at Oban and Killin Junction.
The last two new lever frames to be installed on the line, at Oban Goods Junction and Callander East, included levers to operate detonator placers on the running lines immediately outside the signal boxes. The detonator placers at Oban Goods Junction were originally interlocked with the running signals reading over them (it is not known if this was the case at Callander East).
Manson's tablet exchangers were provided outside most signal boxes between Callander and Oban in 1903.
A fogman's repeater, comprising a miniature fishtailed arm, was installed at the base of the Down Distant signal at Dalmally.