Urban rail transit is a cornerstone of modern city infrastructure, ensuring efficient and sustainable mobility. As cities expand, the track systems that underpin rail transit have evolved to meet increasing demands for safety, reliability, and performance.
Follow this article to explore three primary track systems used in urban rail networks—steel wheel-rail systems, rubber-tired systems, and magnetic levitation (maglev) systems—detailing their structural characteristics, key components, and real-world applications.
Steel Wheel-Rail Systems
Overview
Steel wheel-rail systems form the backbone of most urban rail transit networks, including subways, light rail, suburban rapid transit, and trams. These systems rely on a conventional structure comprising rails, track beds, turnouts, and a variety of fastening components.
Track Bed Configurations
Enhancements: Some systems incorporate vibration-damping features using rubber supports, floating slabs, or dampers to minimize noise and vibration.
Ballasted Tracks:
- Composition: A bed of crushed stone, gravel, or slag.
- Applications: Common in depots, parking areas, and some surface-level urban lines.
Characteristics: Good drainage and shock absorption; however, they require frequent maintenance.
Ballastless Tracks:
- Composition: Reinforced concrete or steel structures replacing traditional ballast.
- Applications: Predominantly used on mainline corridors for modern urban transit.
- Advantages: Superior stability, durability, and reduced maintenance.
P2.Wuhan Metro Line 19
P3.Ballastless tracks
Key Components
- Rails:
Modern systems favor seamless welded rails that minimize expansion gaps and enhance ride quality. - Rail Joints and Fasteners: Secure rail connections are maintained using clips, anchors, and other fastening mechanisms that ensure geometric integrity under dynamic loads.
- Sleepers (Ties) and Track Bed: Sleepers support the rails and maintain proper gauge, while the underlying track bed distributes loads to the foundation.
- Auxiliary Devices: Additional components—such as anti-climbing devices, rail braces, and gauge rods—help maintain stability and safety throughout the system.
Seamless rail construction is predominantly used on mainline tracks, with expansion joints reserved for turnouts, switching areas, and depots. Transition sections are often employed to ensure a smooth interface between ballasted and ballastless segments.
Rubber-Tired Systems
Overview
Rubber-tired systems represent a modern alternative to traditional steel wheel-rail designs.
By employing pneumatic tires, these systems offer enhanced traction, reduced noise, and a smaller physical footprint—ideal for elevated or automated transit applications. They are implemented in various formats, including:
- Straddle monorails
- Suspended monorails
- Automated guided transit (AGT) systems
- Medium- to low-speed maglev variants
- Rubber-tired metros
P1&P4 Shanghai Metro Pujiang Line
Straddle Monorail Track Structure
Design:
Vehicles ride atop a single, robust guide beam that serves as a load-bearing structure, a guiding track, and a conduit for utilities such as power, signaling, and communication.
- Beam Types:
- Precast Prestressed Concrete (PC) Beams: Cost-effective and easy to construct.
- Cast-in-Place Concrete Beams: Customized for specific structural needs.
- Steel Guide Beams: Preferred for long spans due to their flexibility.
- Steel-Concrete Hybrid Beams: Combine material strengths for enhanced performance.
- Support Mechanisms:
- Cast Steel Tensile Bearings: Offer high tensile strength and fatigue resistance, accommodating thermal expansion.
- Rubber Bearings: Lightweight and excellent at damping vibrations.
Suspended Monorail Track Structure
Design:
Vehicles are suspended beneath a single elevated track, which comes in two main configurations:
- Beam-Girder Tracks:
Constructed using steel box girder or I-beam designs, these tracks offer robust load-bearing and lateral guidance capabilities.
Typical spans range from 20 to 30 meters, with a maximum span of 80 meters.
- Cable-Supported Tracks:
Employing suspension bridge principles, these tracks use tensioned cables to support the running surface, allowing spans up to 300 meters between towers.
- Suspension Types:
- Asymmetric Suspension: Components arranged on one side, typically with I-beam structures.
- Symmetric Suspension: Vehicles are supported symmetrically by a steel box girder that integrates essential systems such as power and communications.
Automated Guided Transit (AGT) Systems
- Design Characteristics:
AGT systems feature separate running surfaces and guidance rails: - Running Surface: A concrete slab that carries the train’s weight and transmits loads to the supporting structure.
- Guidance Rail: Provides lateral stability and directional control, positioned centrally or on the sides based on design requirements.
- Standards and Features:
- Concrete Strength:
- Underground Tracks: Minimum C35 grade.
- Elevated/Surface Tracks: Minimum C40 grade.
- Expansion Joints:
Installed every 12.5 meters on tunnels and surface lines, with high-bridge joints matching the modulus of the supporting structure. - Surface Friction:
Must maintain a friction coefficient of at least 0.85; heating measures are often used in cold climates to prevent icing.
Magnetic Levitation (Maglev) Systems
Overview
Maglev systems are at the forefront of urban transit innovation.
By eliminating physical contact between the train and the track through electromagnetic suspension, these systems achieve high speeds, minimal friction, and reduced maintenance.
P5.Maglev Track
Track Structure and Features
- Construction:
Primarily constructed using precast prestressed concrete beams; steel or hybrid steel-concrete structures are also employed as needed.- Configuration:
Many maglev systems feature dual-track beams that share a single pier or bridge structure, optimizing space and structural stability.
- Distinctive Characteristics:
- Seamless Integration: Ensures smooth, continuous levitation even at high speeds.
- Embedded Electromagnetic Systems: Coils within the track provide the necessary forces for suspension, propulsion, and guidance.
- Precision Construction: High accuracy in construction is critical to maintain the flatness and continuity required for safe operation.
Conclusion
Urban rail transit is continuously evolving to meet the demands of growing cities, and the track systems at its core are a critical element of this evolution. Steel wheel-rail systems remain the established standard, providing robust performance and proven reliability. However, rubber-tired systems offer advantages in noise reduction, traction, and space efficiency, especially in elevated or automated environments. Meanwhile, maglev technology is paving the way for a future of high-speed, low-friction travel.
The careful selection and optimization of these track systems—tailored to specific urban conditions, operational loads, and climatic challenges—are essential for ensuring the safety, efficiency, and sustainability of metropolitan transportation networks. As technological advancements continue to drive innovation in track design and construction, urban rail transit will remain at the forefront of modern urban mobility, powering the future of our cities.
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