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Quainton News Archive - Quainton News No. 98 - October 2007
Permanent Way and the NNR - Bob Wright
This article featured in the Spring 2007 edition of the North Norfolk Railway's magazine and is featured here with their kind permission. The components and geometry that make up the permanent way are described with reference to the NNR but apply equally to Quainton. Ed.
The permanent way
The permanent way is the most visible part of a railway, but beneath it is a sub-structure supporting the track, which is made up of two elements, the formation and the ballast.
The formation is the ground on which the track structure is built. This may be either the natural ground or an embankment or cutting surface. The formation has to be strong enough to carry the loads of trains. On the NNR we are fortunate in having a strong gravel formation in most places. The ballast allows the loads from the sleeper bottom to be spread over a wider area. The only ballast strictly needed for this purpose is a pyramid shaped block beneath each sleeper. The remaining ballast provides weight to hold the track in alignment. Where weaker soils exist then a thicker layer of ballast may be used to spread the weight over a larger area. Ballast should ideally have an angular shape to allow it to lock together for stability and to reduce maintenance. It should also have sufficient voids within it to permit good drainage; without this the formation can become waterlogged leading to wet beds where it has turned to liquid mud. Today ballast is generally of granite but historically was of limestone or blast furnace slag. The Holt extension was laid using local flint stone of rounded shape. This was done for economy but requires a higher level of maintenance to keep the alignment correct.
The permanent way itself is made up of the sleepers and the rail. Sleepers are of two types, timber and concrete. New timber sleepers are normally of Douglas fir, pressure treated with creosote. However, in the past these have been of a variety of hard and softwoods including oak and Baltic redwood. The standard sleeper is 8 ft 6 in long, lO in wide and 5 in deep but in the past 12 in wide sleepers were used either side of rail joints. Concrete sleepers have become normal in new works since the 1950s with the adoption of pre-stressed concrete technology. William Marriott experimented with reinforced concrete sleepers at Melton Works, but these were not a success. During the Great War, the shortage of timber led the Great Eastern Railway to produce an alternative sleeper using pairs of concrete sleeper blocks connected by a steel bar. These were used in sidings and lasted into recent years. The concrete sleepers on the NNR, in the Weybourne - Holt section, are BR el type and have chairs fastened by through bolts. Problems have been experienced with some of these where the bolts touch exposed steel reinforcing strands within the sleeper, resulting in a loss of track circuit current.
Bullhead rail was standardised quite early and defined by British Standard BS 9. Nonetheless the railway companies adopted a variety of weights of section, between 80 and 100 lb per yard. The prewar standard for the London and North Eastern Railway was 85 lb per yard rail for branch lines and sidings and 95 lb per yard for main lines. However, rail replaced from main lines during relaying was often re-used on secondary routes. Rails are joined using pairs of cast steel fishplates. The bolts in these are a loose fit in the rail's holes, which permit the expansion and contraction of the rails. The gaps between rail ends vary from 1/8 inch above 75°F and 3/8 inch below 40°F.
Rails are supported in cast iron chairs, which are screwed or bolted to sleepers. The casting includes the maker and date. Surprising, out-of-context examples may sometimes be found, the result of adhoc replacement in the past. The rail is secured into position by keys of timber or spring steel.
A point is properly termed a switch and crossing (S & C). These consist of the switch blades and the crossing assembly. The blades are planed to a taper to provide a close fit to the stock rail. Either side of the crossing area, wing and check rails are provided to assist the guidance of the wheels through the crossing. The switch blades are fixed to each other by a tie bar to ensure that when one is against its stock rail, the other is fully clear and will provide room for the wheel flange to pass through cleanly. S & C timbers are known as bearers rather than sleepers. The supporting chairs for an S & C are varied to suit the angle of the crossing and the presence of checkrails. Those supporting the switchblades have a greased sliding surface to allow them to move freely.
S & C is described by the length of the switchblade, A to E, and by the angle of the crossing, 1 in 6 and greater - A6 is a very tight radius used very rarely, see the accompanying drawings.
Those on the NNR vary between B8 (7 ft 4 inch), 7.1 degrees and C14, versions. Longer switches and flatter crossing angles result in a longer length of unit but permits trains to take the diverging track at higher speed, although this is not too significant on the NNR. Shorter units are best suited to confined locations such as sidings and yards. For example in the new sidings for the Bridge Road Carriage Shed, we will make use of three B8 units to squeeze in the four sidings needed.
Historic permanent way
For the construction of the Yarmouth and North Norfolk Railway, between Stalham and North Walsham, the rails were 70 lb per yard fastened to sleepers with spikes. The sleepers were half round, 9ft long and 9 in wide. The ballast was sand, about 8 in thick beneath the sleepers. This would be a very light construction by modern standards. The use of spikes into thin sleepers would have been hard to maintain and the sand ballast would not have drained very well and it would have been difficult to maintain the line and level of the track. However this form of construction was not unusual in the 1880s on lightly used lines. The growth of traffic would have resulted in the need for heavier construction and most surviving photographic evidence suggests that standard bullhead rail construction soon became the norm.
The permanent way requires much maintenance to keep in it good order. The main defects that develop are described here.
The use of steel wheels on steel rails results in the gradual wearing away of the railhead. On straight track this is usually even between rails, however on curved track the outer rail can become worn on the inside of the head, especially where there are high line speeds. Mainline routes provide railhead lubricators in such locations to reduce this problem. On the NNR the low speeds and reasonable radii mean this is not a significant problem.
The rails on the Railway are 50 to 60 years old, but because traffic has been light they are not too worn. Nonetheless an annual renewal programme is being established to replace a short length each season to ensure that the line is still fit for use in the long-term. Fishplates if not regularly lubricated can rust against the rail and break as the rail expands in the spring. Timber sleepers, despite their preservative treatment, do eventually rot. We are fortunate that our ballast and formations are dry and this has prolonged their life. Concrete sleepers have a long life although some of ours were cracked during construction, which has allowed water to rust the reinforcement leading to the bursting of the concrete.
In order for footplate and permanent way staff to refer to specific locations, railways have always provided mileposts. These are normally located in the Up side cess and give the distance from the main terminus, in our case from South Lynn. Gradient posts are also provided at significant changes in gradient. Their numerals refer to a 1 in x gradient.
Railway companies had an obligation to fence their property to exclude cattle and prevent trespass. The normal fence was timber post with seven wires although lesser specification was acceptable where no cattle existed. The modern railway with high speed trains and the risk of litigation from trespassers has increased the standard of fencing to chain link and in many urban areas to rigid steel palisade fencing. The NNR is generally enclosed with seven-wire type fencing on timber posts, apart from a small length of M&GN style timber fencing at Sheringham.
Text © Quainton Railway Society / Photographs © Quainton Railway Society or referenced photographer
Page Updated: 01 December 2017