Early Design Aspects of Light Rail Systems

Written by Mohammed Reza Zolfaghari, Felix Schmid and Piers Connor

Selected from, “Designing And Managing Urban Railways” by P Connor, N. Harris and F Schmid (Eds.), 2015. 

Background

Horse omnibuses came into existence in the first half of the 19th century, when the world’s major cities began to grow beyond a size where any destination could be reached easily on foot in about an hour. However, horse omnibuses were uncomfortable and had limited capacity, due to the poor road surfaces. The operators had to charge relatively high fares because they had to cover the cost of the two or more horses required to haul the coaches. Between 1850 and 1880, recognising the importance of faster and more efficient local transport and the advantages of the steel wheel on steel rail, entrepreneurs and the municipalities themselves started to build basic street railways, modelled on the tramroads that had emerged in the 17th and 18th century for the carriage of freight. Early trams, as the vehicles came to be known, were hauled by horses and, in some cases, small enclosed steam locomotives or underground cables. Over the years, Berlin, London, Paris, New York and many other major cities rapidly developed large networks of animal, steam and cable hauled tramways.

The cost of horses, with feed, shoeing dung removal and the need to have at least two or three teams to provide an all-day service by each coach led to systems being electrified from 1890 onwards. 

Richmond Va Tram 1888 1

Figure 1: The first viable electric tramway system was built under the direction of Frank J. Sprague in 1888 in Richmond, Virginia, USA. It was 12 miles long and was built along existing streets, including some significant gradients. Its success led to the adoption of electric tramways in over 100 cities in the US within 10 years and quickly spread to Europe.








Electrification provided lower running costs and improved performance and quickly, electric tramways were built around the globe, often by American and European commercial interests, with time limited concessions. However, many of the smaller networks were abandoned in the 1920s when the concessions expired while others were taken over by the local municipalities. Most of the major international conurbations had abandoned their tramway networks by the 1960s because of the need to renew track and electric systems. They were encouraged by the lower cost of diesel fuel and tramways were replaced by trolley-buses and motor buses.

Tramways became popular again in the 1980s, both in Europe and the United States of America. In the USA they reappeared as Light Rapid Transit, an affordable system of medium capacity, fast urban transport that suited the relatively low density of many American conurbations better than metros of the type built for New York City, Chicago and Washington DC. In Europe, the terms Light Rail and Light Rail Transit were adopted in some cases to differentiate the new systems from the long established tramway with its reliance on sharing most of the alignment with road transport. The terms tramway, light rapid transit and light rail are often used interchangeably.

Planning

Whatever the terminology used, the requirements for building new street running railways are now far more demanding than those that were in place in the late 19th and early 20th centuries. The planning phase of modern systems often exceeds 10 years from concept to opening. Usually, a project starts with a feasibility study.

Nott Tram Int

Figure 2: An integrated tramway station in Nottingham, UK showing the arrangements for a tram operating on a public street. This was part of the original NET system. Source: Author.









A feasibility study is an essential first step in any tramway construction project. Any stakeholder or consideration neglected in this phase may result in major flaws in the system and bears the risk of investing public funds in a venture that is rejected by the potential users. To ensure the economic viability and sustainability of a tramway system, a successful feasibility study must identify correctly the areas with the optimum population in terms of size and ability to pay fares, the possible commuting demand and the potential for the creation of strategic transport links. 

The main concept for a successful network should be to link three main types of zones, including central business districts, residential areas and the main public transport hubs, such as railway stations, bus stations and airports. The layout and location of each area, the passenger flows between them and economic justification are the main parameters that can will affect the network planning process. The key to reaching the required passenger flows is the correct identification of areas and creating viable links between them. Another key factor for a successful feasibility study is a determination of the possible contribution that the tramway network might make to urban regeneration. Nottingham and Sheffield are examples for this. Following the success of the first phase of the Nottingham Express Transit (NET) system, the second phase is now adding to the contribution that the tram system makes to the Greater Nottingham economy and its social fabric. Some of the benefits of a well-planned light rail system can be:

  • Significant financial benefits for local companies involved in construction contracts;
  • Employment of hundreds of local people;
  • Growth in employment, potentially creating 8,000 jobs;
  • New investment in the city region with a value of £300m annually;
  • Contribution to an integrated public transport system for Greater Nottingham;
  • Facilitation of modal shift from private cars to public transport;
  • Improved access to about 1270 out of 55,000 work places in the city;
  • Greater integration with road transport leading to reduced congestion and lower carbon emissions.

The foundation for predicting reliable levels of passenger flows is the proper identification of areas to be served by a new system, with data generated from linking traffic zones presented in an origin/destination matrix. This matrix can potentially include information on all types of travel by various modes but tramway network planners tend to focus on transfer from car travel. This is because the car reflects the most convenient and demanding mode and the highest passenger volume and it helps the planners to be on the safe side. The approach also helps the planners to get the data for potential trips within allocated traffic zones with high passenger demand. Also, planners have to convince promoters that there will be enough passenger volume by demonstrating that they have chosen the best routes and alignments. The planners must be aware of passenger flow variations that can be categorised into seasonal, daily and hourly flows. They must allow for occasional markets, seasonal events, seasonal jobs and that university teaching periods and vacations can affect the number of trips and passenger flows. 

Routes

Undoubtedly, route selection and alignment are important steps in the process of building a new tramway. Route design can profoundly affect the time and cost of the project and can impact on the vehicle design, system safety and traffic management. During the feasibility phase, planners should take into consideration the possibility of using existing alignments. This is usually one of the best solutions to address financial and environmental issues and will lead to savings in cost and time in many situations.

Nott Tram Seg

Figure 3: A segregated section of the Nottingham tram system showing the separation of the guideway from the road system and the precautions taken at road crossings with special grids to the side of the road. Source: Author.










The designs for a right of way for a tramway can be mixed, depending on the street layouts of the area to be served, the traffic patterns and the safety requirements. Tramway alignment types can be described as: Segregated, Separated and Integrated. A segregated route is one which is completely protected from interaction with roads, cars and other traffic (Figure 3). This type of alignment is mostly applicable to situations where tramways or light railways are built on land that is not alongside existing roads and other transport infrastructure. It could even be built on former railway routes, such as in Croydon, England. A separated route is a type of tram alignment where the tram track is separated from other traffic by a fence or kerb but adjacent to the carriageway. This arrangement is often referred to as segregated on-street running.

An integrated alignment is a type of route that partially or totally shares the road with other traffic modes such as buses, cycles, motor cycles, cars and trucks (Figure 2). Not unreasonably, planners are keen to design segregated routes wherever possible, since this potentially avoids a number of safety issues, complicated traffic management plans, specific signalling equipment and, to some extent, extra time and cost in the construction process. But there are  some inevitable factors that get in the way of designing segregated routes like topography, street alignments, historical buildings or political and legal restrictions and these make the design of tramway systems in city centres a big challenge. There is also the big issues of the requirement to move utilities, especially those that are not mapped or that appear during construction.

Construction

There is no doubt that construction of a new urban tramway is going to be disruptive and it is important that the project team takes the time and effort to make people and businesses aware of what is to happen and to mitigate the inconvenience as far as possible. At the beginning of the construction of each section of track, installing barriers and blocking some of the pathways are inevitable. Depending on the location of each individual business, the nature of the business and the income and the number of customers of each business, blocking the access affects the business to a greater or lesser extent. For example, some small takeaway shops that have only a small number of customers tend to be at risk of losing all of them. 

New-offices-MRZ

Figure 4: Construction of the track for Birmingham's Midland Metro system extension to New Street station. This shows works after street closure and the disruption to the access for the local offices. It is essential that careful planning and mitigation for this type of disruption is carried out before the start of the construction phase of a tramway project. Source:  Mohammad Reza Zolfaghari.





 

The construction process will include ground excavation, demolition and noise as utilities are moved and track laid. The noise and vibration can transfer to the surrounding buildings and in some cases, offices overlooking the works have had to be abandoned. For large businesses, deliveries often arrive on a daily basis and road closures during tramway works can seriously affect access. It may be necessary to find an alternative route along parallel streets to provide truck access and for rubbish collections. This process requires precise traffic management and, in some sections, variable traffic management systems may be required. Implementing such systems will require lengthy risk and safety assessments and approval processes and these will result in higher costs and longer timescales for the project. 

Experience has shown that, in cities like Birmingham and Nottingham, the approvals process profoundly affects the management of the project and its cost and time budgets. In some projects, the contractor had to split the track construction into separate sub-sections. This had the potential to affect the quality, integrity and resource allocation strategy of the project. It also led to problems for people and local businesses because of multiple changes to the delivery and access routes and their effects on traffic management. It is essential that a comprehensive study is carried out prior to starting the construction of a tramway in a city centre to determine the consequences of the construction process and the priorities of track development in order to achieve the optimum traffic management, utility diversion and mitigation of environmental impacts, such as noise and vibration.

Track

The choice of track system and its design will be an important part of the system design process. The interface between track and vehicle is influenced by a range of factors. According to the US Transportation Research Board (TRB, 2012), in specifying a tramcar, the following requirements must be taken into account:

  • Weight of vehicle in both empty and fully loaded condition;
  • Clearance issues including required track-platform location tolerance, clearance between cars and adjacent tracks, car dynamics, clearance for bridges, tunnels etc.;
  • Gap between vehicle door sill and platform edge that affects wheelchair access;
  • Wheel dimensions including the tyre profile, wheel diameter, gauge and back to back gauge;
  • Lateral component of vehicle forces on track;
  • Longitudinal vehicle/track interaction forces.
Roadway-clearance-ORR

Figure 5: A sample design criteria drawing produced by the Office or Road and Rail in the UK. This example shows the clearances required for two tramcar vehicles passing on a roadway. Source: ORR.











Track geometry and curve radius and length will directly impact the choice of tramway vehicles and their design. The most important parameter in track design is consistency so the same standards should be applied across the network. 

It should be noted that the form of track potentially affects the clearance envelope. For instance, ballasted track can drift during its life-cycle and this can affect the kinematic envelope. All track, vehicle and control system designs should be coordinated from the first stage of design. A good approach is to produce a comprehensive manual for design criteria and then update the manual by adding information as the project progresses. 

Electrification

In the UK, the ORR (Office of Rail & Road) is responsible for regulating all types of rail systems. For tramway electrification, the ORR has some provisions and guidelines that must be followed by developers. In Britain, tramways are generally supplied with electric traction power via overhead lines. The maximum allowed voltage on public roadways is 750 V DC. Different voltages might be permitted in special cases, subject to technical and safety agreements but AC power supply is normally not permitted for on-street systems. Overhead lines require support structures along the route where the electrification equipment cannot be suspended from roadside buildings. These structures and the associated overhead electric traction equipment require careful siting to avoid hindering traffic on the streets and at the same time they need to be safe from possible damage by road vehicles. Different types of poles are used to support the overhead wires, according to location and local conditions. The designs should be carefully chosen to provide the necessary support whilst not being intrusively ugly. 

The designs of poles should at least provide protection against climbing and must be secured against vandalism. The poles in public areas must be insulated properly and there must be provision for electrical protection in case of pole failure. Minimising the vulnerability of any single supporting equipment or pole needs to be a major goal of the design since the collapse of any single support can potentially impose excess tension in the overhead line system. The design must be able to prevent live equipment from dropping lower than 5200 mm above the road . In the case of off-highway rights of way, the limit can be lower but only if it is safely out of reach of pedestrians. Connections between the pole and the contact wire must be physically weaker than the contact wire to guarantee that in case of a pole being damaged or collapsing, the connection will break before the live equipment is pulled down.

Electrical Protection

The design of any electric traction power supply system must guarantee that the maximum touch voltages do not surpass 60 volts. Since the positioning of electrical sub-stations can be potentially significant, minimising the return resistance by the use of sufficient rail section or additional return current conductors must be taken into account. Higher voltages might be permitted in low-risk areas, subject to specific inspections and considerations but access by the public is always a concern.

Isolating switches must be located so as to give the operators effective and efficient control of the power supply system, even in emergency conditions. The switches must be protected from intrusion by unauthorised persons and located without creating any kind of hazard. Isolators are best  positioned in secure trackside cupboards. Where switches are mounted on trackside poles, sufficient protection must be in place. There should be adequate signage to inform people to keep away from live parts. Staff must be trained to use this equipment safely, bearing in mind the public arena. In some areas, depending upon the owners’ consent and safety requirements, electric overhead lines or equipment might be mounted on the roof or walls of adjacent buildings. In these cases, the proximity of the buildings to the equipment, the location of windows and the possibility of touching equipment during cleaning activities must all be considered carefully with a view to ensuring safety.

Stray Current Management

The design of the electrical supply system must take into account the need to minimise stray current. In understanding the process, we may consider the load model for a tramway power supply as consisting of the substation from which the direct current  at the selected nominal voltage for the line is supplied, the positive conductor (the overhead line) connecting the supply to the trams, the load (the tram cars) and the negative conductor over which the current is returned to the substations. According to Ohm’s law, in any conductor the passage of an electrical current will result in a voltage drop along its length, proportional to the resistivity of the conductor. As a result of this phenomenon, sections of the rails will be at a voltage, which is different from that of other equipment buried in the earth, such as metal pipes and cables and urban utilities.

LRT-earthing-ORR

Figure 6: This schematic shows the ORR suggestions for the design and configuration of tramway power supply systems to avoid stray current flow. The voltage is dependent on the location and power flow within the system and could be positive or negative, depending on the local earth. Since the earth is not a perfect insulator, the potential difference between the rails and adjacent buried conductors causes that part of the traction return current to ‘leak’ to earth and some of this total leakage to return to the substation, in part, through the buried pipes. The level of leakage depends on the relative total resistance of the many possible return paths, through the rails and through other metallic pipes, earth shields of cables and the ground itself. Source: ORR.



The impact of stray currents on buried metal objects is known as electrolytic corrosion. This mostly results in loss of material. Undoubtedly this is an unwanted phenomenon in locations where metal pipes carry gas or water,

since leakage and rupture will impose very significant safety and operational risks to the tramway, people and the environment. With regard to utility cables, especially old lead covered types, such losses arise even

more quickly and can lead to failure of the unprotected inner insulation of the cable. It is important that utility companies and other organisations which own equipment buried in the ground beneath the track, accept that, even with a proper design and satisfying all the requirements, there would still be some leakage. Hence the buried equipment and devices must be protected properly. Constant monitoring and maintenance are key factors to assure control and mitigation of return current losses. To begin with, a stray current strategy must be implemented at the very start of the project planning stage. The most important steps in this strategy are:

  • Agreement on the establishment of a register of vulnerable assets.
  • Awareness of programmes where the utility companies replace the cladding of equipment currently metal with plastic or similar, therefore reducing the range and extent of vulnerable devices;
  • Agreements on the level of testing of measures to mitigate stray current throughout the construction phase of the system;
  • Agreements on the level of monitoring of change in these measures during the operational phase of the system;
  • Appraisal of the outcomes of testing during construction;
  • Investigating the issues as they arise during the operational phase of the system;

There must be regular meetings with stakeholders, legal bodies, constructors and companies during the construction phase. In the operation phase these meetings can be less frequent but must still take place regularly to monitor stray current performance.

Conclusions

The experience gained in recent years from new tramway designs in the UK and elsewhere leaves no doubt that careful planning both in the design and construction phases of a tramway project is essential if the system is to be delivered to specification and within the time and budget agreed. Regular communication with stakeholders is vital in ensuring compliance with requirements and with obtaining a satisfactory period of construction with a minimum of disruption and delay. A step by step approach is always a good way towards reaching the ultimate goal of a well run and successful project.

Sources: 

Alkubaisi, M. I. T. (2014) ‘Predefined Evaluating Criteria to Select the Best Tramway Route’, Traffic and Logistics Engineering, vol. 2, pp. 211-217, 2014.

Gassel, C. (2012), ‘Cooperative Traffic Signals for Energy Efficient Driving in Tramway Systems’, 19th ITS World Congress, Vienna , Austria, 2012.

Lesley, L. (2011), ‘Light Rail Developer’s Handbook’, J.Ross Publishing Inc., 2011.

Steel, A. (2014), ‘Design Standards Stray Current Management’ in Tramway Technical Guidance Note 3, ed. http://orr.gov.uk: ORR, 2014.

TRB (2012), ‘Track Design Handbook for Light Rail Transit’. The Transportation  Research Board, Washington DC, USA, 2012.

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