Plain track
I will describe the overhead wiring for a section of plain track without any point work, bridges, stations etc.
Stagger
The wires on plain track are held above the line normally by cantilever mast, arranged so that the contact wire zigzags as it goes along. This is called stagger, and is arranged in this fashion to create even ware on the pantograph of the locomotives and multiple units. The cantilever mast are positioned opposite each other on twin track, there is a push off assembly which holds the contact wire away from the centre line towards the centre of the two tracks. Above the other track is a pull off assembly that pulls the contact wire towards the outside of the track. The distance from the centre line of the track and the position of the contact wire at the registration point (where it is held by the mast) is 9ins for Mk1 OHLE and 230mm for Mk3. If working in 4mm scale this is 3mm.
Contact Wire Height
With Mk3 equipment the contact wire height is 4.7m, slightly lower than Mk1 which is 16ft or 4.864m. This height can vary where there is level crossings, overbridges, depots and yards. With early overhead schemes the height was greater in station areas as well. All this variations will be covered later.
Mast Spacing
The distance between each mast is called the span. Masts are spaced the maximum distance apart where possible. The maximum design span is 73m (958mm in 4mm scale),and the minimum is 13m (170mm in 4mm scale). Were mast are of unequal distance apart the difference between them must not be over 20m (262mm). There are many governing factors, which determine the distance for each span, these include the physical layout of the railway, where the points and junctions are, over bridges, under bridges and viaducts, stations, signals, gradients etc. Another important factor on the real railway is side wind, not a problem with model railways but one worth considering if you are modelling a line going across exposed fells or the fens. When the span is being calculated the “blow off”, sideways displacement of the contact wire due to strong wind should not allow the wire to go outside the pantograph sweep and the two loose contact with each other. So in areas of high cross winds the spans are reduced in length.
Types of Overhead
Fixed Termination Equipment
This is found on older systems like the GER suburban lines and where wire runs are short, e.g. terminus stations. The contact wire is pre tensioned and is supported either end by heavy anchor mast. This type of equipment is preferred on tight curved track as balanced weighted equipment cannot overcome the loads imposed by drag from the pantograph.
Automatic Tensioned Equipment
This more commonly found and uses balance weights each end to keep the tension preventing the contact wire from moving out of place due to temperature changes, wind, and displacement by the pantograph of a moving train. In the centre of each length of wire is an anchor point, without this the wire would creep towards one balance weight due to being pulled along by the pantograph.
Tension Length and Number of Mast
On automatic tensioned equipment the maximum length is 1970m, from the anchor mast in the centre to each overlap at the ends there should be 14 masts, if equally spaced this works out at 70.3m. Where the mast are closer than this the wire length is shorter, the number of mast is the governing factor not the length of wire run until 1970m is reached. Crossovers, sidings, loops etc have shorter lengths with less than 14 masts from anchor point to balance weight.
The distance between the overlaps on the GER suburban lines is approximately 1 mile. There are 28 masts from one end to the other. Due to the contact wire being fixed at each end there is no need for a mid point anchor.
Using the above information if your model layout has no points, obstructions etc and the track is straight then provided the number of mast is less than 14 the overhead will be cantilevers set as to give a stagger spaced at a maximum of a scale 73m. Most model railways are not that simple, we like our stations with their sidings, loops etc. We also like bridges and tunnels, and most of us have sections of curved track.
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