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Section 24: Gradients

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The gradient of the railway trackbed is rarely constant over any considerable length, and it may change several times within the space of a mile. Gradients that are particularly steep may have an operational impact on the working of some trains.

Gradient posts are placed, facing the track, at each location where the gradient of the trackbed changes. Most gradient posts comprise a pair of arms, one on either side of a post. The arms are inclined up or down to indicate the direction of the slope, and each one has the value of the relevant gradient marked on it [24.1 & 24.2]. Gradients are expressed as a ratio, e.g. "1 in 200" (or "1:200"). For example, "1 in 500" means that the track rises (or drops) one unit for every 500 units travelled horizontally. Note that a smaller number in the second part of the ratio indicates a steeper gradient. Some patterns of gradient post omit the first part of the ratio, the value of 1 being implicit [24.3].

[24.1] Gradient Post (e.g. Travelling from left to right, the gradient changes from 1 in 105 falling to 1 in 228 falling).
Area: Various   Usage: High   Status: Obsolescent
[24.2] Gradient Post (e.g. Travelling from left to right, the gradient changes from 1 in 330 rising to 1 in 450 falling).
Area: Various   Usage: Low   Status: Uncertain
[24.3] Gradient Post (e.g. Travelling from left to right, the gradient changes from 1 in 500 falling to 1 in 600 rising). Click Here for Photo
Area: Various   Usage: High   Status: Current

If the trackbed in one direction is level, the corresponding arm of the gradient post is horizontal and bears the word "level" [24.4] or just the letter "L" [24.5]. A level gradient can alternatively be expressed as "1 in ∞", since an infinite number of units would need to be travelled for the track to rise or fall by one unit [24.6].

[24.4] Gradient Post (e.g. Travelling from left to right, the gradient changes from level to 1 in 50 falling).
Area: Various   Usage: High   Status: Current
[24.5] Gradient Post (e.g. Travelling from left to right, the gradient changes from level to 1 in 158 falling).
Area: Various   Usage: High   Status: Obsolescent
[24.6] Gradient Post (e.g. Travelling from left to right, the gradient changes from 1 in 85 falling to level). Click Here for Photo
Area: Great Eastern Railway   Usage: Medium   Status: Obsolescent

The Highland Railway installed separate gradient posts on either side of the line, each showing the gradient in one direction only [24.7].

[24.7] Gradient Post.
Area: Highland Railway   Usage: Medium   Status: Obsolescent

As an alternative to a pair of arms, the direction of the slope can be indicated by inclined bars on a rectangular board. The gradient details can either be displayed above each bar [24.8] or superimposed on them [24.9].

[24.8] Gradient Post.
Area: Various   Usage: High   Status: Current
[24.9] Gradient Post. Click Here for Photo
Area: Lincoln area   Usage: Low   Status: Uncertain

Special forms of gradient posts could be provided at junctions where the gradient of the diverging branch line differs from that of the main line. The gradient at the start of the branch line may be indicated by a gradient post with a single arm projecting in the direction of the branch [24.10]. A gradient post comprising a single arm centred on the post provides confirmation of the main line gradient through the junction [24.11]. Exceptionally, where a junction coincided with a change of gradient on the main line, a gradient post could be provided with extra arms showing the branch line gradients [24.12].

[24.10] Gradient Post for a Junction (showing the branch line gradient).
Area: Various   Usage: Medium   Status: Current
[24.11] Gradient Post for a Junction (showing the main line gradient).
Area: Various   Usage: Medium   Status: Obsolescent
[24.12] Gradient Post for a Junction (showing main and branch line gradients).
Area: Various   Usage: Low   Status: Historical

Where necessary, a gradient post may be fitted with an additional plate indicating to which line or lines it refers [24.13].

[24.13] Gradient Post (e.g. refers to E&F Lines).
Area: All Areas   Usage: Medium   Status: Current

Special precautions were often necessary when working unfitted freight trains down steep inclines, to avoid the train getting out of control. The Great Western Railway was the first company to introduce a practice that required these trains to come to an absolute stop at the top of the incline. The guard would then manually apply a sufficient number of wagon brakes to keep the train under control on its descent. After descending the incline, the train would stop again; this time, to allow the wagon brakes to be released. A worded notice board was provided at the top of the inclines where these procedures had to be carried out [24.14 & 24.15]. Other companies that adopted the same practice installed boards with alternative wording, whereas the LNER provided signs with just a letter "B" for "brakes" [24.16]. Later signs had the letters "AWB" for "apply wagon brakes" [24.17 - 24.19]. At some locations, a standard stop board (see [26.14]) was used, with the wording "Stop - Freight train wagon brakes to be applied".

[24.14] Incline Stop Board (e.g. Up direction trains).
Area: GWR   Usage: Medium   Status: Historical
[24.15] Incline Stop Board (e.g. Down direction trains, with 35 wagons or less).
Area: GWR   Usage: Low   Status: Historical
[24.16] "B" Sign.
Area: LNER   Usage: Medium   Status: Historical
[24.17] "AWB" Sign.
Area: Elgin - Keith   Usage: Low   Status: Historical
[24.18] "AWB" Sign.
Area: London Midland Region   Usage: Low   Status: Obsolescent
[24.19] "AWB" Sign.
Area: All Areas   Usage: Medium   Status: Obsolescent

The Severn Tunnel is over four miles in length, with long steep gradients. A short section of level trackbed near the middle of the tunnel is approached from the Welsh end on a 1 in 90 falling gradient and from the English end at 1 in 100 falling. Marker lights are fixed to the tunnel walls in both directions, indicating the approach to the change of gradient. These originally comprised a single white light [24.20] at a quarter of a mile followed by two white lights [24.21] at 40 yards from the point where the falling gradient changes to level. To maintain a tight coupling, drivers of freight trains descending the gradient were required to release the brakes gradually before passing the single light and apply power before reaching the double lights.

Additional lights for reverse direction moves were installed in 1989 when bi-directional signalling was commissioned through the tunnel. In 1992, the colour of the lights was altered from white to blue [24.22 & 24.23].

[24.20] Gradient Marker Light.
Area: Severn Tunnel   Usage: Low   Status: Historical
[24.21] Gradient Marker Lights.
Area: Severn Tunnel   Usage: Low   Status: Historical
[24.22] Gradient Marker Light.
Area: Severn Tunnel   Usage: Low   Status: Current
[24.23] Gradient Marker Lights.
Area: Severn Tunnel   Usage: Low   Status: Current

Owing to difficulties experienced with the disc brakes fitted to Class 158 DMUs in severe weather conditions, special instructions were introduced in 1995 requiring drivers to regularly test the brakes whilst driving. In addition, special signs depicting a snowflake symbol [24.24] were provided on the approaches to Druimuachder and Slochd Summits (between Perth and Inverness), and subsequently also at Beattock Summit on the West Coast Main Line. During freezing conditions or when it is snowing (or when fallen snow may be disturbed by passing trains), all Class 158 DMUs are required to make a full service brake application and stop at these signs before proceeding.

[24.24] Running Brake Test Sign.
Area: Scotland Zone   Usage: Low   Status: Current