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Where art meets engineering

Resembling the rim of the new 12-sided GBP1 coin, crossing the centre of this high-level view is the outbound Second City Crossing (2CC) track at St Peter’s Square in Manchester. It embodies three curved sections and two straight sections. What could justify ‘breaking the back’ of this curve in two places like this? No bus would ever follow such an odd wheelpath. The inbound 2CC curve (nearest the camera) consists of one large natural curve. Why couldn’t its outbound neighbour be the same? (Image credit: Matt Doran)

There is more to tramway engineering than the essentials of getting vehicles from A to B. The appearance of track and overhead line equipment (OLE) can be enhanced so that it looks stylish and contributes to the city’s ambience and becomes a civic asset – art takes many forms, so why shouldn’t tramways be one of them?

While few may specifically notice, any more than they go around in their daily lives admiring buildings, I hope that this article will show that good-looking tramways can be more cost-effective than unsightly ones and how being easy on the eye adds to the feel-good factor.

More than a century ago, elegance was lovingly designed into tramways and infrastructure contributed greatly to civic pride, instead of being just a collection of functional clutter. Where traction poles were necessary they were, in the style of the day, decorated with extravagant scrollwork, spiky finials and ornate cast iron bases as well as being painted in attractive colours instead of the plain black or grey of today’s popular supposition. The track embodied gracefully-shaped transition curves which looked, and felt to the passenger, far more natural and efficient than the clumsy trainset-like joining of sharp circular curves directly to straight track or, worse still, via ugly and troublesome kinks.

Shiny steel rails are an attractive advertisement of the mode and are expressive of the tram’s ability to combine the best elements of railways and tramways. Looking good, as well as adding to their appeal, also happens to be related to technical excellence and whole-life cost reduction.

This exploration begins in Den Haag more than 30 years ago, when I was made painfully aware of tram track geometry after being launched violently across the standing area at the back end of a long articulated unit as it was accelerated fiercely out of a sharp curve. Even being relatively fit and able to hang on tightly, in a lapse of concentration I was flung against a stanchion so hard that my ribs were bruised. Years later I was surprised to see the same type of tram about to enter the same curve on the front cover of a Permanent Way Institution treatise on best practice for tram track design and construction.

Shortly after the UK city of Manchester announced its tramway plans, it became clear that wheelchair users would be catered for, along with all the other beneficiaries of improved access for more ambulant disabled people; senior citizens; pregnant women; and parents with small children – it is worth remembering that every one of us starts life with no personal mobility. On the other side of the country, the Tyne & Wear Metro was already showing the way with multi-accessibility, and it became evident that improved access wasn’t going to be just a question of getting wheelchairs or pushchairs on and off light rail vehicles. Welcoming a wide range of passengers would imply a need for a new focus on journey quality by providing as smooth and event-free a ride as possible.

Elderly people and children, for example, may lack the strength to resist powerful dynamic forces, and their reflexes may not be fast enough to react quickly to sudden jerks in unpredicted directions. People with walking aids have their own difficulties holding on, and they won’t always find a seat. Buses can unfortunately subject elderly, disabled, pregnant and other passengers to a cocktail of unpleasant dynamic events. The potentially far more civilised journey quality of a tram can, with careful engineering, ensure that riders are able to relax, knowing that they are not going to be thrown around the passenger saloon.

It should be emphasised that, while initiatives aimed at improving access and ride quality may be intended primarily for the benefit of people with reduced mobility, all other passengers benefit – although they may not always realise that it is often lobbying by the mobility-handicapped which has gained enhancements for everyone.

It does have to be said, though, that if high platforms are adopted rather than low floors for light rail services, they may not be especially street-compatible. This might mean that desirable stops have to be omitted or even done away with, as in Manchester, where Mosley Street lost its popular outbound stop. I would contend that there really ought to have been one more stop on the new Metrolink Second City Crossing (2CC), which like the lost Mosley Street stop would have relieved the busy interchange at St Peter’s Square. An additional stop on 2CC would, moreover, equalise the 1CC/2CC running times between St Peter’s Square and Victoria, simplifying service planning and balancing the network core.

Keeping it smooth

Lateral dynamic disturbance is related mainly to cornering characteristics – curve radius, superelevation and curve shape are key factors. There are several textbooks dating from as long ago as the start of the 20th Century with formulae for determining the precise shape of curves. Inside the PWI publication there were photographs showing transitioned tramway curves, yet the text included the words “the need to provide transitions for passenger comfort are minimal”. Why is it that some of today’s practitioners seem so ambivalent about track geometry and its relationships to improved accessibility and to track maintenance?

It may be that there is a fixation with low tramway speeds in city centres and with the tram’s ability to negotiate contortions in track. The rules of physics, however, don’t entirely take a holiday at low speeds and the design and execution of tram track needs to be future-proofed as it is far more permanent than anything that runs on it. To this end, track distortion should never be used as a speed-limiting tool. That is the job of comprehensive staff training and supervision and, possibly in the near future, of GPS-based electronics such as SIMOVE (TAUT 950).

The reverse curve which takes Metrolink trams outbound from St Peter’s Square across Oxford Street is a good example of what can be achieved. Frequent double units need to clear the busy road crossing quickly so as to keep the traffic moving and the naturally-flowing track geometry enables the trams to accelerate freely away into Mosley Street without disturbance to passengers. On the double inbound tracks here, trams can overtake each other, and with no surface markings it is easy for pedestrians to be confused about the directions from which trams will approach. This brings a conflict – trams on all three tracks need to get a move on to clear the road and pedestrian crossings quickly, but in the interests of safety they ought to go more slowly.

If the pedestrians are ever protected by signals, and if the trams are ever fitted with a more effective audible warning of approach, and if overtaking by trams is ever prohibited there, it may be permissible with caution to increase the line speeds slightly. That is where the smoothly-flowing track geometry will really come into its own.

The techniques for refining track geometry are closely related to those used by the designers of roller coasters. In the case of theme park rides, the objective is to design thrills into the system; for public transport, one of the aims should be to design thrills out of the system. Passengers don’t want their knuckles whitened on their way to work, and neither do they want their journey to be tiresomely sluggish – so why isn’t dynamic comfort at reasonable speeds designed into every aspect of every tramway system?

The role of transitions

Why is fixed-track geometry so important? Manual steering, even on dodgem cars, can’t accommodate instant changes of curvature because of the time taken for a steering mechanism to operate; for the same reason, cars, lorries, buses or ships can never follow a circular path throughout any change of direction.

On main line railways, changes of curvature are always gradual enough to accommodate cant gradients in which the outer, or ‘high’ rail, is raised higher than the low rail. Cant is applied so as to tilt the train and offset the cornering force so that passengers (and their soup) are not subjected to undue lateral forces.

It is rarely possible to build optimum tilt, or cant, into street tramways because the tracks must lie flush with the roads in which they are embedded. Having said that, today’s profusion of humps and potholes makes one wonder (tongue-in-cheek) whether road surface distortion may have become less unacceptable these days…

In any case, there may not always be enough ‘give’ in the articulation and suspension systems of modern trams to provide the twist tolerance needed to allow them to ‘lean into’ superelevated curves, and the partial unloading of individual wheels must be avoided in the interests of minimising derailment risk. An absence of superelevation does not, however, cancel out the desirability of easing entry into and exit from every curve of whatever radius and at whatever line speed, if only for the sake of preventing sidewear unevenness.

Late in the 19th Century, tramway engineers began inserting transition sections, or easements as they are sometimes known, at either end of curves to enable trams to enter and leave them smoothly – at junctions, transitions were cast into the special trackwork. Sudden sideways shocks to trams, track and passengers were reduced in severity, and the ‘angle of attack’ of wheel flanges against rails was evened out through curves, eliminating localised gouging and reducing both the whole-life cost and the risk of flange-climb.

Furthermore, when the speed of general traffic increased during the following decades, trams could keep up instead of quaintly lurching their way round contorted corners. This is part of the future-proofing of good tramway engineering. Another benefit of flowing track geometry is that it subtly enhances the aesthetic appeal of tram track as circular curves abutting directly to tangents look unnatural, as well as being technically inferior.

If transition curves, preferably with prescribed shapes such as spirals of an appropriate length, are specified, curve quality can be secured through rigorous supervision. Failing to specify curve shape incurs a risk of geometrical anarchy taking hold, wherein the practitioner takes advantage of the freedom to get the track from A to B without regard to future issues around maintenance or passenger comfort. That is just one way in which kinks can be inflicted upon us. Certain standards must be reliably delivered, and that can’t just be left to chance.

Transitions were once especially effective with two-axle trams in which the leading wheels, through the axlebox hornways, rigidly transmitted all lateral shocks directly to the entire mass of the tram and its passengers. The whole cornering force was transmitted from the rails to the passengers, equipment and bodywork – a poor state of affairs from an engineering and customer-care viewpoint if curves lack transitions at either end. Most London County Council trams, including the M class four-wheelers, had swing bolsters in an attempt to soften the effects of entering curves. The need to maintain momentum when negotiating junctions with gaps in the sub-surface conductor rails meant that London conduit trams tended to enter curves fast, and that made it necessary to soften the process with whatever means possible. Swing bolsters and transitions helped, and bogies in any case reduce lateral shocks because of their ‘virtual transition’ property, with the swivelling point of attachment to the bodywork some way in rear of the leading axle.

That was the situation in the heyday of tramways, but the laws of physics don’t change, and the engineering principles prevailing today are not different to those applying on the vast city tramway systems of yesteryear. Running gear configurations are constantly evolving, and not all of them feature conventional bogies with their self-contained virtual transition properties. Single axles and the steering of individual wheels are strengthening the need for transition curves to ease entry into and exit from all curves by all trams, present and future. One thing is certain – transition curves are far more compatible with technological developments than untransitioned, or kinked, curves can ever be.

Feature originally published in June 2017 TAUT (954).