Fahrzeuge und Gleisbau:Trassierung/en

Roadbed and Survey

One common thing to most light railways is that they run for short distances only.
Furthermore a light railway is always fitted to the existing landscape, thus avoiding complicated structures along the right of way. So track-curvatures and grades are dependant on the locale of the layout. Any earthwork is limited to the absolute minimum necessary.
On construction of a light railway system there is mostly one thing you must pay attention to, the clearances: The proportion of the clearance width at 2100mm (6' 10-5/8") to the gauge at 600mm (23 5/8") is quite high on light railways. In addition between clearance and any fixed objects a further 500mm (20") is to be kept free. And finaly there are several cars and engines exceeding the max. width of 2100mm (6' 10-5/8"), especially when they had been designed for wider gauges. Therefore the clearance on 600mm gauge light railways may also be set to the dimensions of 750mm (30") gauge clearances.

The survey of a light railway's layout is quite simple due to the short distances and the fact that the low speeds don't afford too high standards on construction. This on the opposite does not mean, that you simply might take a few frame-tracks, put them on ground and bolt them together. This may work on temporary trackage and with low car-weights, with permanent trackage a sound construction will reduce later maintenance.
When laying out the direction of the tracks it is absolutely sufficient to take orientation on prominent spots in the landscape. As mentioned above, it is only necessary to look for the clearance, which is easily accomplished with a ruler. If your survey runs across open, level ground you can make use of homemade steering-poles for orientation.

To measure the width of gauge you need a track-gauge and a spirit-level. The track-gauge can simply be scratchbuilt from steelbars, setting the measure to 600mm (23 5/8"). When working on curves, the gauge has to be widened, which can easily be accomplished by inserting additional flat steelbars. The widening of the gauge increases with tougher radii. This bears the potential danger that narrow wheels will drop between the rails.
Special attention has to be paid to the back-to-back distance of the wheelsets and span of track (with a gauge of 600mm [23 5/8"] no narrower than 540mm [21 1/4"]). This applies when using girder-rail and mounting guard- or wing-rails. The rolling stock of light railways is often from various suppliers and in some case even includes regauged equipment. So the wheelsets and especially the flanges vary a lot. Therefore you must pay attention that the flanges don't sqeeze on the inside. With girder-rail you also have to check out the depth of the flangeway to prevent the wheels from running on the flanges.

If possible, on railroad construction you try to avoid any kind of grade. On the opposite it is typical to light railways, that they are fitted to the landscape rather than making use of bridges or tunnels.
If a grade is inevitable, it is to be kept to a minimum degree, since the friction between steel-wheel and steel-rail is considerably lower than the friction between tire and pavement, and dependant on the axleload which is quite low on light railways. Furtheron the friction is reduced by oil, water and greasy dirt on the railhead. Therefore grades on railroads are usually measured in per thousandth making 15 per thousandth a steep grade. If extreme grades where encountered, light railways made use of cable-inclines.
It is advisable to construct as continuous a grade as possible. Also you must pay attention to a thoroughly rounded change of grade, not allowing any kinks. On light railways this is best accomplished with those not too popular short pieces of rail.

The Roadway

The roadway of a light railway can be kept simple. Nonetheless a certain amount of care should be employed, not only laying the track on the ground as it comes. Important to the roadway is a ground being as even as possible and well drained.
Construction should start with removing the turf, even on level ground. Then you should equalize any uneven patches. On this graded roadway you put a layer of fine gravel. On light railways ballast was hardly used, for ballast is a lot more expensive than fine gravel. Nonetheless the gravelbed must have a certain minimum thickness, it is recommended to take half the distance between two ties as a minimum for the gravel depth under the ties. If you have a unusualy watertight ground, as often found around clay pits, it might be advisable to remove the earth in the width of the roadbed and to the depth of .5m to .8m (20" to 32") and replace it completely by fine gravel.
No matter if fine gravel or ballast is used for the roadbed, it is essential to tamp the roadbed thoroughly. The usage of machine-tampers, no matter which brand, is highly recommended.