Transit Technology: Fundamentals and Subsequent Development Rev. 2/2/8/2001

                        Pictures & more facts of this technology

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.

So the compartments can deal broadly with track and truck, top or bottom body suspension, power supply system from lineside to vehicles, control and propulsion system on board, signaling, field reporting and automation, auxiliary systems for heating, lighting, communications and air conditioning, train performance station spacing and fleet requirements.

Article #1 : Trucks and track, rubber tires, steel wheels, maglev.

1) The steel wheel/steel rail combination

The first railroad wheels comprised assemblies of hubs, spokes, and rims, the whole held together by a steel tire, with the tread and flange to guide the train along the track. This arrangement derived directly from carriage and wagon practice. Running on cast iron rails, the combination permitted heavier loads, less rolling resistance, and more weather-proof operation, than was known on the roadways at that time. Later the wheel center was cast as a single piece, but the rim was still shrunk on, to be replaceable for wear and maintenance. Most recently, the entire wheel is a single steel casting, large enough to withstand being turned several times to restore the profile, before returning to the manufacturer for scrap and recasting.

The introduction of steel wheels in place of cast iron created a much stronger and wear resistant combination, so that permissible weights of rail vehicles can be as much as 32tons/axle. It is not essential that rail vehicles should be heavy. Some light rail systems have not more than 5tons/axle, and still function well.

The special characteristic of steel wheels in railroad and subway practice derives from the fact that they are mounted solidly on the steel axle, making an assembly known as a “Wheelset”. As the vehicle moves along the track, both wheels turn at identical rates.

The treads of the wheels have a taper, known as “coning”, usually a slope of 1 in 20. There is only one line in the circumference of each wheel, where the radii of the two wheels are identical. When rolling along the two rails of a track, the wheelset will roll in a straight line, if the rails are exactly  under  these equal diameters.

In the event of any lateral displacement of the wheelset, the outer wheel now runs on a diameter larger than that of the inner wheel. Since both wheels must turn at the same rate, the outer wheel moves further forward than the inner, naturally curving the wheelset towards the inner wheel. If the track is still straight, this brings the wheelset back to the central point, so that the vehicle continues to follow down the track. This is the self-steering characteristic of the railroad wheelsets. Under normal circumstances, the coning alone keeps the train on the track, and the flange is never needed to contact the rail head.

If the line is entering into a curve, the natural curving of the wheelset will follow round, as long as the coning allows the appropriate difference in wheel diameter, to match the curvature. The outer wheel would run on the larger diameter, and the inner wheel on a smaller diameter.

If the curve in the track is more than the natural curve of the taper, then the leading outer flange will press against the outer rail, and force the wheelset inwards, sliding across the head of the rail. The sharper the curve, the greater this sliding must be. The amount of slide compensates for the difference between what the natural curve would be, with the flange just against the rail head, and the actual amount of curvature.

It is this lateral sliding that generates vibrations and noise when rail vehicles are taken round curves sharper than the tapered wheels can accommodate. Sky Train has registered patent disclosures describing methods of enhancing the natural curving of standard railroad or light rail wheelsets.

1b) Performance in severe climates.

The contact point where the wheel rests on the rail is very small creating a high pressure area. This means that any ice that accumulates on the rail cracks off easily, allowing adequate traction when the train runs over it. The power of the propulsion system usually is sufficient for a train to force its way through two or three feet of snow. In areas of the country where snow falls can be heavier than that, there are snow plows, pushed by locomotives, that clear the tracks for regular trains to follow behind. Sometimes freezing rain obstructs the movement of the switches, so that switch heaters are required, to melt the ice off, and allow service to continue. If ice builds up on electrical contact surfaces, it can interrupt the currents for propulsion or for signals and control. In areas of the country where winters are particularly hard, special measures are taken to prevent any build up of ice on electrical surfaces, that will ensure the continuity of service, regardless of weather conditions.

In some countries, railroads have constructed roofs over switches in main lines, to keep them clear of  snow and freezing rain. When the tracks and switches of an overhead suspended system lie within a monoduct, the duct affords natural protection against the weather.

1c) Signaling systems and automation

There is now an experience of almost 100 years with railroad signaling systems based upon the fact that the steel wheel/steel rail combination can report automatically the presence of a train, by way of the wheel sets connecting the two rails together electrically, that be detected by track circuits at line side. There are many different systems using this principle, all of which have demonstrated complete safety and reliability.

The most recent developments have been to install communication and control in both directions, from train to wayside, and from wayside to train. This uses the signal system to operate the train automatically, so that the risk of human error is eliminated, and safety of the train movements builds upon the same reliability. the same principles are applied in high speed rail operations, at speeds up to 500km/h, and to subway and rail commuter operations, for speeds up to 90km/h and stations spaced only one or two kilometers apart.

2) The pneumatic tire

2a) Early development

Until the coming of the automobile, in the late 1800’s, highway transportation was either by horse, mule, or horse-drawn carriage or wagon. The wheels were assembled from hubs, spokes, and sections of rims, the whole banded together by steel rims, heated and shrunken to hold the whole assembly tightly together. There was no elasticity in the wheel, and sometimes, even no springing in the whole vehicle. Roadway transportation passed over rough roads, mud, compressed earth, stones or crushed rock. In the absence of heavy rolling machines, the roadway surfaces developed ridges, grooves and wash-boards, that made travel very uncomfortable. Only the wealthy afforded leather springs, and later leaf springs, to ease the roughness of travel.

At the time the first inventors tried to put out the earliest self-propelled road vehicles, steam or gasoline driven, these vehicles made greater speeds than ever before, and travel over the roughness of the roads clearly became impossible, without some technique for damping the shocks. Solid rubber tires came first, then some thicker rubber tires had  holes in them, to allow some flexibility. There were also wheels with flexible rims, with springs all around inside them, and rubber tires round them. The first automobiles followed the tradition of light pony carts, with spoked wheels and thin, solid-rubber tires. Light springing was included, but speeds had to be held down to comfort levels.

The solution came with the pneumatic tire. This was effectively a balloon wrapped round the wheel, covered with a tough outer casing, to resist cutting by the stones of the roadway. The tire yielded to the minor bumps, and removed the small vibrations. Unfortunately, they were not big enough to overcome the bigger ridges, troughs and wash-boards. So a powerful public pressure group developed, the “Good-roads” associations, crying for improved roads, that lead to the first “puddled” roadways. This was where crushed rock was rolled in with a heavy steam roller. It did not take much traffic to destroy these surfaces, and potholes soon developed. Further development created the asphalt surfaces known today, and poured-concrete expressways soon followed.

There was a parallel development in the pneumatic tire. As first developed, tires were small, thin, and needed a relatively high pressure of air inside them. Larger, heavier and faster road vehicles needed larger tires, designed to carry greater loads, to be able to travel at high speeds on good-quality roadway surfaces, yet tolerate some roughness in the country lanes. So we have the larger, low-pressure tubeless tires of modern times.

There are many different models of modern tires, ranging from bicycle tires to road vehicles, through automobiles and pick-up trucks, to heavy tractor-trailers. Each tire has a designed load-carrying capacity, that should not be exceeded, for risk of catastrophic failure.

A problem with the pneumatic tire is the low pressure of contact on the road surface. The least build up of ice or snow on the roadway results in loss of adhesion, skidding and loss of control of road vehicles. The capacity for good acceleration and braking is greatly diminished, so that vehicles are difficult to start moving, and equally difficult to stop once they are moving. Highway authorities operate highway snow plows, or spread salt to melt the snow and ice. Never the less, winter conditions often bring rubber tired operations to a complete standstill.

2b) Rubber tires in transit services.

In the early 1950’s, attention was being directed to the significance of noise, and one target was the noisy railroad. Streetcars and subway trains rattled and clanked along, but it had always been that way, so designers had not taken any need of noise suppression in their design thinking.

Now pressures were on, to demand less noise from highway traffic, so attention was turned also to the steel rail systems. In France, experiments were made with pneumatic tires on subway trains, that were adopted on three or four of the Paris Metro lines. These tires need to be inflated with a high pressure of 165lbs/sq ins of nitrogen. The same systems were exported to Montreal and to Mexico City. A general conception developed, that the way to achieve more silent service would be to abandon steel wheels altogether, and use rubber everywhere. The other claim to advantage was that the rubber tire offered better adhesion than the steel wheel/steel rail combination.

Subsequent concepts for transit systems frequently have proposed to use rubber tires, as an automatic assumption that this was the way to go.

Unfortunately, this assumption brings with it the limitations on permissible carrying capacity of the pneumatic tire. This was no hardship, when the objective was to offer a small, light, low-cost system, for special applications, and low traffic levels. Many of such proposals have been with this objective in mind. To try to use it in high-performance, high carryings transit systems, the combination is expensive in first cost and maintenance costs,.

Another limitation of the pneumatic tire derives from the lack of physical contact between the vehicle and the roadway. Signaling systems to ensure train separation, require other, and less secure, methods of determining train location, for information from the field to the signal system, and commands from the center to the train, to ensure proper response in train movement. In the French metro system, steel rails are still required, to carry the trains through the switches. Since the rails have to be there anyway, they are used also for signaling. Steel shoes beneath the trucks rub on the rails, to report the presence of the train, and allow central control to direct the traffic movements.

The sensitivity of the pneumatic tire to winter conditions is a hazard for transit. Buses in city streets, or on the open road, are susceptible to the same delays and risk of accidents as other forms of road vehicle. In the winter conditions of Montreal, transit trains on rubber tires are found solely in the subway underground. Nowhere does the operation pass into the open air.

Incidentally, the pressure for quieter operations so much influenced the design of steel wheel vehicles, to the extent that the subway in Washington, DC, is so quiet, that it needs to have lights at the platform edges, to worn waiting passengers that a train is entering the station.

2c) Steering of rubber tired vehicles.

Horse drawn vehicles on the roads had a separate foot board, riding on the front axle, where the shafts were attached. The vehicle body rested on a fifth wheel on this foot board, similar to that of a semi trailer truck today. To turn a corner, the driver moved the reins to lead the horse round. This brought the axle round, so that the vehicle had a natural steering from the horse leading it into the curve

The first “Horseless carriages” used the same principle, except that the driver had to turn the front axle with a tiller, similar to that controlling the rudder on a ship. This was a cumbersome method, and soon gave way to separate front wheels, linked  with a steering bar. This arrangement persists today on motor vehicles.

Automated vehicles with electric propulsion on rubber tires have been created with a steering mechanism similar to this, as in Dallas-Fort Worth airport. The cars run in a channel, with low walls on each side. In front of the vehicle, there is an arm on each side running against the wall of the channel. Each arm is linked to the steering mechanism of the front wheels, so that the vehicles follow round curves by natural steering. Such an arrangement makes it to be essentially a one way vehicle in the forward direction.

When rubber tires were to replace steel wheels in subway service, this kind of natural steering was not available on the rail tracks. The two fixed axles on a truck under a subway car have no steering mechanism, and would roll naturally in a straight line. To follow a track round a curve, it has to be forced round. On the Paris system, side walls were installed along the tracks, and the trucks were provided with horizontal wheels in front and behind, to press against the walls for guidance. In the Westinghouse system, the guidance is by horizontal wheels pressing against a central steel beam. Photographs of a system being installed in Jacksonvile, FL, show a central concrete beam for guidance in a similar way. In these cases, the guidance is provided front and back, so that the vehicles and trains can be operated in either direction.

3) Maglev

This word is an abbreviation of “Magnetic Levitation”. There is a certain magic in the thought that magnetic force might be used to lift a vehicle into the air, and move it along a track, with no mechanical contact or wearing parts. Modern electronic components can control the flow of electric currents, also without moving parts that wear out and require maintenance at specific intervals.

Powerful magnetic forces can be generated in three different ways, by attraction, by repulsion, and by motoring forces on conductors carrying current in a magnetic field. Each of these methods has been proposed for systems, but none is currently in commercial operation.

3a) Levitation by attraction

When a powerful magnet acts upon a soft iron armature, a force of attraction develops. The distance between them governs the magnitude of the force, so that the closer they are, the stronger is the force of attraction. The result is an instability, such that the magnetic force takes control, the two parts jump together, and the magnetic gap reduces to zero. This form of magnetic attraction could not be adopted for use in a system of maglev for vehicular movement, without some way of regulating these forces.

A technique for regulating these forces has been under development in Germany since early 1960, and is now at the stage of proposing to construct an inter-city line between Hamburg and Berlin. The specialty is a sensor, that senses the distance between an electro-magnet and the armature, and adjusts the current  to maintain a constant gap.

Krauss-Maffei in Germany has applied this technique to levitate vehicles on a guideway. The guideway comprises a central beam, with continuous projections or wings on each side. The soft iron armatures are continuous strips attached along the undersides of these wings. The vehicles ride above the beam, and have arms that reach down each side, to curve under the armatures. The lifting magnets are attached to these arms, so that, when energized, the magnets are attracted upwards towards the armature, and the body is lifted upwards. The sensors ensure that the magnetic gap does not close completely, and they adjust the magnetic force to just support the body weight..

The magnetic gap is small, so that the vehicle cannot be lifted more than a short height above the beam. This could be a disadvantage for potential applications in countries having severe weather conditions, with snow and freezing rain.

A vehicle floating on a magnetic field has no mechanical contact with the guideway, so some alternative form of propulsion is needed to accelerate the vehicle and later bring it to rest again. Proposed systems generally have used linear motors in the track, both for propulsion and braking. These require a control system on the ground, to activate coils in the linear motors, according to the actual speed of the vehicle, and the desired acceleration or braking effort. The control system requires knowledge from the field, as to the actual speed and location of the vehicle in motion, to be able to energize the motor coils at the proper frequency in the proper  zones. The claim is made that there is no known limit to attainable speeds on a maglev system, and record speeds have been demonstrated exceeding 500km/h. A further claim is made that maglev can climb grades steeper that conventional steel wheel/steel rail systems, but the claim that steel wheels cannot climb the same steep grades remains to be demonstrated. In both cases, the commercial value of these characteristics remains to be proven.

A different solution is required, for a detection system, that can report the field situation, because the vehicle has no contact with the guideway. This meets a need equivalent to that of a signal system in the conventional railroad or subway operation. Detection currently depends upon various forms of block system. Detectors sense the passing of the vehicle into the next block, and pass this information to central control. Here action is taken to activate the linear motors in the next following block, that the vehicle will react to, upon entering it.

The other lack in a non-contact system is in the provision of on-board power, to activate heating, lighting, air conditioning and communication systems. This requires either on-board engine-generator sets, or a method of induced power transmission from line side.

Only one commercial installation is known to have existed to date. This was a short link in Birmingham, England, connecting the railroad station with the air terminal. The system rendered service satisfactorily for several years, but was abandoned recently when it could not demonstrate adequate reliability, and required excessive maintenance. Any advantages in attraction maglev as a principle in short-haul and commuter operations remain to be demonstrated.

3b) Levitation by repulsion

The technique of magnetic repulsion has the advantage of being self-regulating. The more the repulsion creates a gap, the less is the repulsive force, so that the amount of lift matches the vehicle weight. Against permanent magnets, the tendency is to reduce the strength of the permanent magnetism. The choice usually is to use electro-magnets.

A new technique is to create the electro-magnetism by means of super-conducting coils attached to the vehicles, that allow very strong magnetic fields. Super-conductors require extremely cold temperatures. The vehicles carry cryogenic devices, either refrigeration systems or liquid nitrogen tanks, that cool the coils by evaporation. In either case, this need involves extra weight, that must be accommodated on board, and adds to the force required to levitate the vehicle.

A maglev system based on repulsion has been in research in Japan for many years. In that system, the levitation is created by induced currents in passive coils embedded in the track. Repulsion lifts the vehicle only at a speed above 50km/h. This requires a wheeled support  or “Landing gear”, for the vehicle to move on, until the acceleration brings it up to speed. There is need also for additional repulsion coils at the side for lateral constraint, since the repulsion forces could allow the vehicle to slip off sideways.

The experimental installation has already broken speed records exceeding 500km/h on land. It remains to be demonstrated that there will be commercial value in repulsion systems offering speeds at this level.

3c) Levitation by motoring forces.

This is a system that is under development in Florida. The full technical details remain the subject of patent applications, and are not available for disclosure at this time.

It is understood that the levitation coils will be embedded in the sides of a pre-cast concrete beam, and will be powered from an adjacent substation. There are to be permanent magnets on the vehicles, that will provide an upward thrust against the magnetism of the coils. This should allow levitation, even before the vehicle is in motion.

The absence of mechanical contact has the same need for linear motor propulsion as in all maglev systems, for sensing and reporting the situation in the field, for control of the levitation and propulsion coils, to ensure vehicle separation and spotting at stations, and to provide on-board power for heating, lighting, air conditioning and communications.