Transit Technology: Fundamentals and Subsequent Development

This is a series of articles by Sky Train Corporation, discussing the foundation and development of some familiar details of transit technology. The text reflects to some extent the separate functions that the different components can serve, a way  of  slotting the ideas into compartments. For example, the size and design of vehicle bodies depends upon the load-carrying capacity of the trucks and track, the size of body that the clearances of the right of way can accept, but also on the nature of the traffic to be carried, such as short haul commuter traffic, long haul passengers, airport feeders, and the various natures of freight traffic. Given these constraints, body design follows basic principles.

Article #2 : Truck and track:  the characteristics of rail gauge

1) History

There is a historic reason why many railroads in the world were built to the peculiar gauge of 1.436m (4ft 8½ins).

When the Romans built their roads, they laid stone runways just wide enough apart for the wheels of the roman chariots, to allow their military forces to travel quickly across long stretches of Europe.. Over the centuries, the passage of the wheels wore deep grooves in these stone runways, that still can be seen today.

With the development of the industrial revolution, in the roman tradition, the first rails were just wooden strips, laid in rows, to make it easier for horses to haul wagons of wood or coal in the coal mines of Britain. The wagons had wheels with a simple flat tread, that had no lateral control to keep them on the rails, while the horse walked in the mud between them.

In those days, there was no treatment for wood against rot, so the life of rails was short, under the constant passage of the wagon wheels, and replacement was an expense  to the mine owners. The natural next step was to place flat metal plates on the rails to keep the wood from wearing down so quickly. Flat plates tend to develop depressed centers and raised-up edges, so there still were ongoing problems. The track maintainers  doing this work were called the “Plate Layers”, a name which remains in use on some railroads to this day.

It was easy to turn up the outer edges of the plates, to keep the wheels running in line, so this was the first example of “Fixed Guideway”. Wagons wheels had an outside dimension about 1.35m (4ft 6ins), so the turned-up edges or  flanges were spaced apart about 1.436m (4ft 8½ins), that became the “Gauge”. A side benefit of turning up the edges was a significant strengthening of the plates, so the track had a much reduced cost of maintenance. Still it was not completely effective, because horse-drawn wagons had the usual steering front axle, so that, if the horse stumbled out of line, the wagon dug into the side of the plates, and could displace them.

A new step was to put the flanges on  the wheels, and let  them run on top of the plate flanges, locking the front axles, and achieving a natural steering, independent of the horse’s actions. The flanges on the wheels had to be spaced apart to run within the plate flanges, leading to the now standard dimensions of the gauge today. A parallel development was the adoption of the solid wheel/axle assembly, with coning on the wheels, giving a strong natural steering, so that operations along tangent or gently curving track did not use the wheel flanges at all. Only on sharp curves do the flanges press against the side of the rails to force the vehicles round the curves.

The first locomotives were conceived to run on this kind of  track, and rapid expansion of the network in Britain quickly established this to be standard gauge, from where it spread across the world, as engineers traveled abroad and brought this standard with them to new areas of rail construction.

2) Choice of different gauges:

Only when railroads were to be built in territory requiring sharp curves, was it recognized that narrower gauge should be brought in, and for higher speeds and stability, wider gauge was desirable.

In the building of railways all over the world, the separation between the rails, known as “The Gauge”, has differed over a wide range, all the way from 0.40m (15”), to as wide as 2.1m (7ft). The gauges most used in various countries, are from 0.90m (3ft) in Africa, up to 1.65m (5’6”) in Russia. There are counter balancing factors governing these choices, according to the nature of the business the railroad is to serve. Comparing the options, narrow gauge makes it easier for trains to go round sharp curves, but does not offer good lateral stability. Trains have lower constraint against falling over sideways, or blowing over in high winds. The opposite applies for wide gauge: lateral stability is much better for speeds on curves and against high winds, but the ability to go round curves is limited.

3) Narrow Gauge:

Narrow gauge places the rails and wheels close together, so that when the track makes a curve, the distance the outer wheel has to roll compared with the inner wheel is small, making it easy for trains to navigate round sharp curves. The narrowness of the support beneath the train brings with it a loss of lateral stability. Trains rounding the curve have a tendency to fall outwards, so the permissible speeds on those curves are limited. Even on tangent track (“tangent track” is standard railroad terminology for a length of track that is absolutely straight), if trains exceed certain speeds, according to the quality of the track, lateral oscillations (or roll) can build up until the cars overturn. The same factors mean that cars on narrow gauge tracks can be sensitive to strong lateral winds, that can become strong enough to lift the wheels off the rail and blow the cars off the track. In Newfoundland, before the rails were abandoned, the rail gauge was 1.05m (3’6”). A certain section of route passed along the tops of cliffs at ocean side, known as “Topsails”, and there was a history of trains being blown off the track on that section. There was a permanent resident there, charged with reporting to rail headquarters when the strength of the wind exceeded limits, in which case train movements would be halted, until the winds subsided.

Since the reason for choosing narrow gauge may be the need for sharp curves, these curves force the use of short-wheelbase trucks, again demanding low permissible speeds, even on tangent track. Short trucks begin to “Nose” as speed rises, so that safe speeds are related to length of trucks, and thus indirectly to the choice of sharp curves in narrow gauge alignments. Some narrow gauge railways have trucks as short as 0.9m to 1.05m (3ft to 3’6”), known as “Square” trucks, because the length is almost as short as the gauge. Even on standard gauge, modern streetcar systems keep to short trucks, not much longer than square, to be able to run round street corners.

If there are not sharp curves, the truck wheelbase could be longer, and the “nosing” of trucks would not be a limiting factor, but this does not overcome the problem with winds on narrow gauge, and the tendency to roll at speed.

4) Wide Gauge:

The wider gauges are chosen for applications where sharp curves can be avoided, as in flat plains and along river beds. The less sharp curves permit long-wheelbase trucks, and thus high permissible speeds on tangent track. On the curves that do exist, the extra stability of the wide gauge allows fast safe speeds on the curves, according to the amount of the curvature, and resists the effects of high winds. When the new subway service was being designed for “Bay Area Rapid Transit”, (BART), in California, a gauge wider than standard was chosen, on the grounds that San Francisco Bay frequently suffered extremely strong winds, and the extra stability was needed to meet them.

The tendency  to roll on tangent track can still be manifest if speeds reach those limits where track is of poor quality, but on good quality track, such limits have not yet been reached. In France, where speed records exceed 500km/hr (312.5mph) on standard gauge, trucks used in both electric power units and cars have a wheelbase more than two times gauge.

5) Sky Train’s choice of gauge:

The intent of the monoduct system proposed by Sky train is to provide damped lateral sway in the secondary suspension, so that cars can swing outwards for high speeds and passenger comfort  when going round curves. At the same time, the intention is to design the system to operate in southern climates, where stability in high winds is a necessity.

Lateral wind forces would tend to push the car bodies sideways, taking weight off the rail on the down wind side. The stability will be afforded by suspending the vehicles beneath the two rails at a wide enough gauge that will support the trucks, and thereby resist wind forces. The damping of the secondary suspension will restrain the rate of swing, and will allow the car body to remain immobile while standing at a station platform.