Railway and Road Engineering: Track Design, Sleepers, and Intersections
Railway Track and Road Design Fundamentals
Slab Track vs. Ballasted Track
Comparing slab track and ballasted track, slab track offers several advantages:
- Low maintenance
- High availability
- Low structure height
- Low weight
Life cycle studies suggest slab tracks can be cost-competitive. However, slab track also has disadvantages compared to ballasted track:
- Higher construction costs
- Higher airborne noise reflection
- Difficult alterations in track position and superelevation
- Limited displacement allowance in the embankment
- Lengthier repair works after derailment
- Transition challenges between ballasted and slab track
Slab Track Variants
Examples of slab track variants include:
- RHEDA 2000
- ZÜBLIN
- Embedded Rail
Switches
Switches are moving parts, also called blades or points, that redirect trains to different tracks.
Concrete Sleepers: Advantages and Disadvantages
Advantages
- Heavy weight (200-300 kg), useful for CWR track stability
- Long service life with good or replaceable fastenings
- Great design and construction freedom
- Relatively simple to manufacture
- Cheap compared to timber sleepers
Disadvantages
- Less elastic than wood; pumping may occur on poor formation
- Susceptible to corrugations and poor quality welds
- Risk of damage from impacts (derailment, loading/unloading, tamping tines)
- Dynamic loads and ballast stresses can be up to 25% higher
Twin-Block Sleepers
Twin-block sleepers consist of two reinforced concrete blocks connected by a coupling rod or pipe. Advantages include:
- Well-defined bearing surfaces in the ballast bed
- High lateral resistance due to the double surface area
Monoblock Sleepers
Monoblock sleepers are beam-shaped with dimensions similar to timber sleepers. Advantages include:
- Lower price
- Little susceptibility to cracking
- Can be pre-stressed
Steel Sleepers
Steel sleepers are rarely used due to:
- Insulation issues
- Maintenance challenges with tampers
- Relatively high price
However, they offer:
- Long service life
- Great dimensional accuracy
- Positive residual value
Cant
Cant is the height difference between the rail running surfaces of the outer and inner rails.
Ideal Cant
An ideal cant occurs when the resultant of gravity and centrifugal forces acts perpendicular to the railway track axis.
Types of Intersections and Exits
- Simple intersection: No island on the inlet or central lane.
- Channelized intersection: Includes at least one inlet on a dividing island or central dividing line; roundabouts are included.
- Public exit: Designated by the road administrator to a facility with business activity (e.g., petrol station, catering, hotel).
- Individual exit: Designated by the road administrator to one or several individually used objects.
Road System Hierarchization
Road system hierarchization classifies roads into levels based on function and traffic capacity. It is essential for:
- Providing local property access
- Collecting local traffic for through traffic
Hierarchization is connected with road designing and planning by designating a Strategic Transport Network.
Transitional Curve
A transitional curve smooths the change between a straight line and a circular curve in roads or railways. It is constructed in the connections between straight and curved sections to reduce abruptness, improving driver comfort and safety. Clothoids can be used for construction.
Street Cross-Section and Intersections
Types of intersections:
- Grade-separated without ramps
- Grade-separated with ramps (interchanges)
- At-grade
Information needed for proper intersection design includes traffic volume, visibility, signalling, turning radii, and right of way.
Factors Affecting Geometric Designs
Factors include topography, speed design, traffic, environmental/economic factors, and vehicle properties.
Importance of Speed
Speed influences traffic safety and efficiency. Engineers control it through curve design, speed bumps, and other measures.
Types of Visibility
Several types of visibility are considered, such as stopping visibility, overtaking visibility, and intersection visibility. Calculations are performed to ensure adequate sight distances.
Human Physiology and Road Users
Human physiology limits road users through reaction time, reduced night vision, fatigue, and speed perception.
Psychological Aspects and Driver Behavior
Psychological aspects such as stress, distraction, aggression, fatigue, and risk perception affect driving.
Sag/Crest Vertical Curves
Mathematical equations considering inlet and outlet slope, curve length, and design speed are used to calculate sag/crest vertical curves. Determining the highest/lowest point is crucial for visibility and drainage.
Traffic Lights
Traffic lights regulate traffic at intersections and improve safety by managing the right-of-way. They are necessary when traffic volumes are high or when there is a risk of collision. However, they can cause unnecessary delays or congestion.