Why Railroads Don't Need Expansion Joints

Summary

This video explores the evolution of railway track systems, focusing on the transition from jointed rail to continuous welded rail (CWR). The clickety-clack sound of old tracks was caused by gaps between rail segments, which were necessary for accommodating thermal expansion and contraction. However, modern CWR eliminates most of these joints, reducing maintenance and improving efficiency. The video explains the engineering behind CWR, including the effects of temperature and stress on rail materials, with demonstrations to illustrate how temperature changes and stress affect the length and durability of materials like steel. It also discusses the challenges of managing rail buckling and the strategies used to maintain safe and smooth railways.

Takeaways

• 😀 Trains used to make a distinct clickety-clack sound due to joints in the tracks, but modern rails with continuous welded rail (CWR) have eliminated most of that noise.
• 😀 Joints in old railways were a solution to moving shorter rail segments and allowing for thermal expansion and contraction in hot weather.
• 😀 Modern railways use continuous welded rail (CWR), which eliminates most joints and results in smoother, quieter, and more durable tracks.
• 😀 Rails still experience thermal expansion and contraction, but CWR tracks use different engineering techniques to manage these changes without creating damaging stress.
• 😀 Continuous welded rail is welded together using techniques like electric flash butt welding or thermite welding, creating stiffer and smoother connections between rail segments.
• 😀 Thermal expansion can be predicted using a simple formula involving temperature changes and a material’s coefficient of expansion.
• 😀 Materials can also expand or contract due to stress, which is known as elastic deformation, and this concept is key to understanding how CWR manages thermal changes.
• 😀 Restrained thermal expansion, a technique used in CWR, works by applying tension or compressive stress to materials to limit their dimensional changes due to temperature.
• 😀 Buckling is a potential problem with CWR, particularly in hot weather, and it can cause derailments, so railways must carefully manage compressive stress to avoid this issue.
• 😀 Continuous welded rail installations aim for a high neutral temperature, ensuring minimal compressive stress in the rails and preventing buckling during temperature swings.
• 😀 The spacing of railway ties and the ballast (crushed rock) play a crucial role in keeping rails from shifting or buckling under thermal stress, ensuring safety and stability.

Q & A

Why do modern trains no longer make the famous 'clickety-clack' sound?

The 'clickety-clack' sound that people associate with trains is no longer common because modern railroads use continuous welded rail (CWR), which eliminates most of the joints in the track. The joints, which caused the sound, were originally used to solve the transportation and thermal expansion issues in track laying.

What are expansion joints, and why were they used in older railroad tracks?

Expansion joints were used in older railroad tracks to allow for the thermal expansion and contraction of the rails. These joints had a gap, which allowed the rail to grow or shrink with temperature changes without causing stress or damage to the track.

What are the disadvantages of using jointed rail compared to continuous welded rail?

Jointed rail is noisy due to the impact of train wheels passing over each joint. It also causes increased wear and tear on the rail and additional stress on the ballast. Additionally, jointed rail can lead to 'rock and roll' in freight cars, creating an unstable ride and increasing maintenance costs.

How does continuous welded rail (CWR) reduce maintenance costs?

Continuous welded rail (CWR) reduces maintenance costs because it eliminates the joints, which reduces wear and tear on both the rails and train wheels. The welded sections are stiffer and smoother, resulting in a quieter and more durable track that requires less frequent repairs.

What is the difference between thermal expansion and elastic deformation?

Thermal expansion is the increase in the length of a material when it is heated, caused by the atoms moving apart. Elastic deformation, on the other hand, occurs when a material is stretched or compressed under stress but returns to its original shape once the stress is removed.

What is the role of elastic modulus in material deformation?

The elastic modulus is a measure of a material's stiffness and its ability to resist elastic deformation. It describes the relationship between stress and strain in the elastic range of a material's deformation and helps engineers understand how a material will behave under different forces.

How does restrained thermal expansion work in continuous welded rail?

In continuous welded rail, the rail is constrained by being connected at each tie, which prevents it from expanding freely when temperatures rise. This restriction creates tensile or compressive stress in the rail, which compensates for thermal expansion and keeps the track stable.

What is 'sun kink,' and why is it a concern for railroads?

'Sun kink' refers to the buckling of continuous welded rail caused by excessive heat on hot days. As the rail expands, it can buckle if there is too much compressive stress. This poses a significant risk for derailments and is closely monitored by railroad operators to ensure safety.

How do railroad companies manage buckling in continuous welded rail?

Railroads manage buckling in continuous welded rail by ensuring the rail is installed with a high neutral temperature, which minimizes compressive stress. They also carefully inspect the rails, especially during temperature extremes, and use methods like restraining the rail to control thermal expansion and prevent buckling.

What is the importance of ballast in maintaining the stability of railroad tracks?

Ballast, the crushed rock beneath the railroad ties, plays a critical role in maintaining the stability of the track. It holds the ties in place, preventing them from shifting horizontally, and helps distribute the load of the trains. It also provides drainage and keeps the track from shifting or buckling.