Concrete sleepers in track construction are a product that stands for continuity rather than innovation. But there is a lot going on in this area. In this interview, Wolfgang Hölzl - Engineering, Design & R&D Sleeper at voestalpine Railway Systems - tells us why sleepers are more than just a simple piece of concrete and what the sleeper of the future looks like at voestalpine Railway Systems.
The sleeper is the central connecting element between the rail, the fastening system, and the ballast. Its most important task is to safely transfer the vertical loads from train traffic down into the ballast. A second fundamental task is to ensure the lateral stability of the track. Temperature fluctuations — especially in summer — create stresses that can cause the track to buckle sideways. With its resistance to lateral displacement, the sleeper ensures that the track remains stable, does not deform, and the train cannot derail. This makes it a safety-relevant component that holds the track and rail in position.
Compared to wood, steel, or plastic, concrete has established itself as the most durable and sustainable material for sleepers. Concrete sleepers have a service life of up to 50 years and are highly resistant to environmental influences. They also have low thermal expansion, which is beneficial for the stability of the track.
Wooden sleepers, on the other hand, must be impregnated to ensure their durability. In the past, substances that are considered harmful to health and have been banned in the EU were used for this purpose. In addition, tropical woods were often used, which is no longer acceptable today for climate protection reasons.
Steel sleepers are mainly used in industrial areas, but they have a poorer carbon footprint and do not achieve the same stability as concrete. Plastic sleepers are a niche product that is mainly used where low weight is important, such as on bridges. Their service life is similar to that of concrete, but recycling is difficult.
The main challenges are vertical load transfer and lateral track stability. When trains travel over the track, dynamic forces from the wheels act on the rails, which are transferred to the ballast via the fastenings and ties. There, they cause wear and tear and destroy individual ballast grains — a key cost driver in railway operations because ballast has to be cleaned or replaced regularly. Larger contact surfaces could distribute the pressure better and thus reduce wear.
However, there are limitations in sleeper design. The dimensions must be compatible with existing track-laying machines, in particular tamping machines, which compact the ballast under the sleeper. These machines work with fixed sleeper spacing and multiple tamping units. If the sleeper spacing were to be changed too much, they could no longer be used efficiently. The challenge is therefore to achieve the largest possible contact surface and thus optimum load distribution within these technical limits.
An important aspect is the choice of cement. Classic Portland cement is very energy-intensive to produce and causes high CO2 emissions. Today, large portions of it can be replaced by slag, which is also produced as a byproduct of steel production. This results in a significantly lower CO2 formula, which we also use in the tracks at voestalpine Railway Systems. We have taken a significant step toward environmental protection.
By 2040, traffic volume on existing lines is expected to roughly double. At the same time, it is virtually impossible to extend track closures and maintenance windows. This means that the service life of the entire track system must be significantly increased. Against this backdrop, voestalpine Railway Systems has developed a new, innovative sleeper and fastening system.
The main focus was on optimizing the sleeper shape. It is significantly wider in the area of the rail seat and has a larger contact surface. This reduces ballast compaction and wear. At the same time, the special shape ensures higher transverse slip resistance and thus better track stability than standard track sleepers. The rail seat is significantly larger, allowing up to four fastening systems per rail. On straight sections, where the loads are lower, the system can be fitted more easily. In tight curves or areas subject to particularly high loads, the fastening can be doubled.
In Germany, longitudinal cracks increasingly occurred in sleepers during the classic manufacturing process with direct bonding. Deutsche Bahn is therefore switching to a process with end-anchored sleepers, in which the tension wires are not tensioned before but only after hardening. voestalpine Railway Systems has developed an innovative process for this purpose, which is also suitable for long-bed production. Deutsche Bahn is already using this process for track sleepers. The switch for turnout sleepers is planned between 2028 and 2030.
Wolfgang Hölzl has been part of voestalpine for 25 years. Since 2015, he has been responsible for research, development, and engineering for sleepers and slab track. He is particularly motivated by the chance to strengthen the overall railway system through structured and goal-oriented research — especially with regard to components that are often underestimated, such as sleepers.