What are railway gradient posts and why are they necessary?

What are railway gradient posts?

Photo: A typical SCR gradient post.
Image 1: A typical
ex-Midland Railway Co.
gradient post.

Gradient posts are installed beside railway lines wherever there is a change in the gradient of the trackbed. Each gradient post within the SCRCA has (or originally had) two arms, one either side of a central support post (see Image 1). The angle of each arm is set to give a clear indication of the direction of the gradient:

  • The arm is angled upwards if the trackbed rises in that direction.
  • The arm is set horizontally if the trackbed is level.
  • The arm is angled downwards if the trackbed falls away from the location of the gradient post.

The angle of the arm does not accurately reflect the severity or otherwise of the gradient. That is indicated by the letters and / or numbers painted, fixed or cast on the arm on the side facing the running lines (tracks):

  • If the track is level, the word 'Level' or the letter 'L' is shown.
  • If there is a gradient, the gradient is given as a ratio in the form '1 in nnn' or just 'nnn' (the latter version being the most common within the SCRCA). The smaller the number ('nnn'), the more severe the gradient.

For more photographs of gradient posts within the SCRCA, see the Structure Type Definition for Gradient Posts.

Why are railway gradient posts necessary?

The short answer is "because most of Britain's railways are not flat". Rather, they undulate significantly, as illustrated by the gradient diagrams below (Figures 1 and 2).

Tip: To open a larger version of these images, click / tap on the thumbnails.

Figure 1

Midland Railway Gradients, Sheet 23: Settle Junction to Ais Gill Viaduct.

Figure 2

Midland Railway Gradients, Sheet 24: Ais Gill Viaduct to Petterill Junction (sic).

The drivers and firemen working steam locomotives need to have a detailed knowledge of the line and its gradients so they can ensure that enough steam is available to power the locomotive at all times, but without wasting coal and / or water.

In the past, the guard on unfitted freight trains needed a similar level of route knowledge so that, just before and immediately after each significant change in gradient, he could manually engage / disengage the brakes in the guards van (and, where necessary, on individual wagons along the length of the train).

The Settle-Carlisle Railway was built with a 'ruling' gradient of one in a hundred (1 : 100). This means that the steepest sections of the route climb or fall one unit (e.g. one foot or one metre) for every one hundred of the same units travelled horizontally. Many sections of the Settle-Carlisle line have shallower gradients and some sections are level. However, the infamous 'Long Drag' is an almost continuous thirteen mile 1 in 100 climb from Settle Junction to the south end of Blea Moor Tunnel. In steam days, this presented an exceptionally tough challenge for both the locomotives and their crews.

The drivers of modern diesel and electric trains still need to be aware of the direction and severity of gradients, especially when driving locomotives that are hauling heavy freight trains.

Acknowledgements

Text and photographs by Mark R. Harvey (© Mark R. Harvey, 2017-2020).

The two gradient diagrams and some background information were kindly supplied by Dave Harris (the Study Centre Co-ordinator for the Midland Railway Study Centre in Derby.

The gradient diagrams were produced for the Midland Railway Company in 1876. They have been uploaded to the SCRCA Project database with acknowledgements to the Roy F. Burrows Midland Collection (item ref. RFB20542) housed at the Midland Railway Study Centre. They are licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. For full details of this license, see:
http://creativecommons.org/licenses/by-nc-sa/4.0/
The two diagrams form part of a bound volume (28 sheets and an index) entitled "Midland Railway Gradients". A photographed copy of the entire volume can be downloaded as a 29.3Mb .pdf file via:
http://www.midlandrailwaystudycentre.org.uk/documents/RFB20542.pdf

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