Why are there crushed stones beneath conventional train tracks, while high-speed rail tracks have none?

The slower, relatively basic green trains of the past have become a memory of the times, replaced by electric multiple units (EMUs) and high-speed rail.

They are called ballast. This distinguishes the type of track: tracks with ballast are used for conventional trains and are called ballasted tracks, while tracks without ballast are used for high-speed rail and are called ballastless tracks.

Both high-speed rail and conventional trains need tracks to run, but why do conventional train tracks have crushed stones underneath, while high-speed rail tracks do not? What is the reason behind this?

Ballast is essentially crushed stone, primarily consisting of rocks such as granite, basalt, limestone, and sometimes even slag as a substitute for traditional stone materials.

Ballast is not simply any type of crushed stone. To meet functional requirements and perform its intended roles effectively, it must possess sufficient strength, and its particle shape and gradation must comply with specific standards. This is because the durability of ballast particles directly influences its overall performance.

Though these impacts may seem insignificant, they can shorten the service life of railway tracks. Over time, subtle changes in track gauge may occur, increasing the risk of derailment accidents.

Ballast helps disperse the pressure exerted by trains on the ground. The construction process of railway tracks involves first laying a layer of ballast on the roadbed, compacting it, and then placing sleepers and rails on top.

Ballast supports the track sleepers, distributing the weight of trains and rails to reduce concentrated pressure. This provides cushioning and minimizes pressure, helping to prevent deformation of the rails and sleepers while ensuring smooth train operation.

Ballast also serves a drainage function. The gaps between the crushed stones form effective drainage channels, allowing rainwater to quickly seep underground and enhancing the overall drainage capacity of ballasted tracks.

This prevents the roadbed and sleepers from being soaked by rainwater or eroded by water flow, reducing maintenance costs.

The drainage properties of ballast also inhibit weed growth on the tracks, keeping the area around the railway clean and tidy.

Just as roads rely on drainage ditches and sewer systems to protect urban streets from heavy rain and flooding—reducing waterlogging—ballast plays a similar role as a "drainage expert" for railways. Without it, the track foundation would be less stable, and the sleepers would deteriorate due to prolonged exposure to rainwater.

Ballast also facilitates track maintenance. The gaps between particles provide space for storing dirt and debris, which can be washed away by rainwater and absorbed into the ground.

If rails and sleepers become heavily soiled due to natural or human factors, their service life could be shortened, leading to additional costs.

However, ballast also requires regular maintenance. Over time, ballast can wear out and become contaminated. Debris such as sleeper fragments, fallen cargo particles, and dust can fill the gaps between stones, impairing drainage efficiency.

Thus, periodic cleaning, screening, and replacement are necessary. During this process, ballast is gradually consumed. Railway maintenance personnel must tamp and compact it, remove fine crushed stones, and replenish it with new gravel.

One reason is that high-speed trains travel too fast, and using ballast could lead to ballast flying.

Conventional trains typically operate at speeds between 80–120 km/h, while China’s high-speed trains generally run at 250–350 km/h.

Imagine passengers standing on platforms less than two meters from the tracks while waiting for a high-speed train. If flying stones were to hit the train windows, they could cause damage; if they were to hit people, injuries would be inevitable.

When French high-speed rail used ballasted tracks, the ballast underwent special treatment—it was bonded with adhesive to form a stable, cohesive mass.

To give an analogy, it might resemble a rubberized running track. Though the bonded ballast retains a granular texture, it forms a unified whole and does not scatter.

First, ballastless tracks provide superior stability. Compared to loose, gap‑filled crushed‑stone ballast, tracks constructed with concrete maintain their geometric alignment much longer and are highly resistant to weathering. Their maintenance costs and required workload are also lower and lighter than those of ballasted tracks.

Maintenance personnel do not need to frequently replace or repair them, which reduces interference with train operations, cuts labor costs, and increases line utilization efficiency.

The concrete‑based track structure conforms to aerodynamic principles, exhibits a more refined form, and ensures that all operational indicators remain within safety standards even under high‑speed running.

As early as 2004, China’s Ministry of Railways built the first ballastless track test section connecting Suining in Sichuan and Chongqing, with a total length of 13.16 kilometers.

Ballastless tracks also feature excellent smoothness and uniform stiffness. They are lightweight, which reduces the load pressure on bridges; have low structural height and require less excavation, minimizing environmental impact; and offer better economic efficiency in tunnels compared to ballasted tracks.

In terms of design, ballastless tracks provide higher bonding quality, facilitate integrated construction, have a long service life, ensure high safety, meet electrical insulation requirements, and can be put into use progressively during construction.

Ballastless Track: Built on a solid concrete base, its millimeter-level surface precision relies on professional grinding to eliminate minor irregularities, ensuring an ultra-smooth ride.

Ballasted Track: With an elastic structure of crushed stone ballast, it is prone to deformation and wear under repeated train loads, requiring regular tamping, stabilization, and profile restoration.

Grinding removes corrugations, maintenance prolongs lifespan. Every precise operation is a solemn commitment to safety and efficiency.

The perfect state of track begins with grinding and is achieved through maintenance.

Today, we see fewer passenger trains on such tracks, with most remaining ballasted lines dedicated to transporting coal and freight. Their wheels roll steadily into the distance, carrying goods across vast landscapes.

Meanwhile, more comfortable and efficient high‑speed rail has become the preferred choice for intercity and interprovincial travel, gliding gracefully on sleek ballastless tracks.

From conventional trains to high‑speed rail, from ballasted to ballastless tracks—this evolution reflects the inevitable progress of our times.