The Iran Rail Industries Development Company, in line with the overarching policies of the Ministry of Roads and Urban Development regarding the increase of speed and axle load to make rail transport more attractive, has undertaken the design and development of up-to-date global systems to reduce the transit time for goods and passengers, as well as to decrease costs and enhance safety. Among the latest products added to the IRID product portfolio, one can mention the design and construction of the rail expansion joint system.
The rail expansion joint system (Figure 1) is a device that allows for the relative longitudinal movement of two adjacent rails in such a way that the track gauge remains unchanged and the path does not deviate.
These longitudinal movements can occur under the following circumstances:
a) In Continuously Welded Rails (CWR);
To achieve high speeds in rail transport and to enhance safety, it is necessary to eliminate the connection joints by welding short rails together and extending the rails. As a result, with the elongation of the rails according to the formula ΔL=α⋅L⋅ΔT (where ΔL is the longitudinal change due to expansion and contraction; α is the coefficient of linear expansion; L is the length of the elongated welded rail; ΔT is the temperature change in the region), the longitudinal changes created in the railway lines increase, leading to the buckling of the rails. Therefore, the use of these systems in CWR (Continuously Welded Rails) lines is very necessary and vital.
B) Structural movement;
Structural movement in bridges and tunnels (Figure 2) occurs through the movement of the bridge piers or columns, which leads to the movement of the rails situated on these structures. Thus, the use of a rail expansion joint system before bridges and tunnels is essential to address the movement defect of the rails.

Figure 1 is an example of a rail expansion joint system.
1- Slide chair
2- Sliding spring clips
3- AT Slide chairs
4- Fixed Stock rails
5- Moveable Switch rails
6- Bearer strap
R- Reference point
C) A combination of the two movements above;
These systems are actually used to prevent any deviation or buckling of the rail due to thermal and dynamic loads in various rail industries around the world. An example of the buckling of railway lines is shown in Figure 3. In turnouts, this buckling of the rails also causes the switch machines to malfunction, leading to the derailment of the passing vehicle and resulting in accidents at the location of the turnouts.
The warping of rail lines and the distortion of track gauge not only cause various accidents to occur, but sometimes, to safely pass through these areas, the speed of the rail fleet must be significantly reduced. This issue also creates numerous problems for the rail industry, as often, even with reduced speed, it is not possible to traverse these regions. For this reason, the presence of this system in railway lines plays an important role in ensuring safe passage and also in protecting the lives of passengers.

Based on the type of application, these systems can have various designs. In general, these systems must be designed in such a way that, in addition to withstanding various thermal and dynamic loads, they facilitate the movement of the moving rails without any disruption to the track gauge, and this issue raises the design and machining challenges of this product. Furthermore, this product, due to being classified as a high-safety component (where any malfunction in the system can lead to unfortunate and harmful incidents), requires greater sensitivity and precision in its design and production.

The Iran Rail Industries Development Company, with its high engineering capabilities, is in the process of manufacturing and producing this system alongside global competitors.