The most obvious feature of the railway track is two steel rails. Then, how did the steel rails evolve into what they are today, and how are they manufactured?
History and Manufacturing of Rails
Birth of steel rails
The birth of the railway can be traced back to the 1650s. In the coal mines of Germany and Britain, the most primitive wheel-rail system was born: the carriage pulled by a mine cart ran on a wooden track. There is no baffle on the wooden track, and there is no rim on the wheel. The car is stabilized on the track by two nails on both sides of the body, which is basically similar to the structure of today’s roller coaster. Later, wooden baffles were added to the sides of the wooden track to stabilize the wheels on the track. Since wooden tracks were not wear-resistant, in 1767 Reynolds developed steel tracks. Since the “steel rails,” this time are directly laid on the long wooden strips parallel to the rails and arranged longitudinally, the sleepers at this time are actually vertical sleepers, which are different from the horizontal sleepers commonly used today. Later, it was found that it was not economical to add baffles on both sides of each rail, and it was sufficient to set up a backing plate on one side of the rail. Therefore, in 1776, Curr developed the L-shaped rail. Due to the drainage problem of the longitudinal sleepers, the sleepers are easy to rot, and the horizontal sleepers were born. In order to increase the strength of the rails between the sleepers, the straight rail design was replaced by a raised in the middle, called the “Fish-Bellied Rail”. later, people realized that if the baffles are set on the rails(Flanged Rail), the entire line must be designed with baffles; but if the baffles are set on the wheels(Flanged Wheel), only the wheels need to be designed with rims, the material required is greatly reduced. Thus, in 1789, Jessop designed a wheel-rail system composed of steel rails without baffles and wheelsets with rims, which became the basis of the common wheel-rail system today. The adoption of the rim marks the official separation of railways and roads, becoming two different modes of transportation.
Rail types evolution
Subsequently, rails with various cross-sections were adopted, and the more representative ones are T-shaped rails and bridge rails（Bridge Rail）. The T-shaped rail is fixed on the base by horizontal bolts, and the shear force applied to the bolts is very large, which easily leads to failure of the bolts because the bottom surface of the rail is very narrow, excessive stress will also cause the base to fail. However, the upper surface of the bridge rail is relatively thin, so it is not very wear-resistant, and even the extreme situation that the upper surface may completely fall off may occur. In addition, bridge rails are difficult to bend horizontally, making the arrangement of curves difficult. Eventually, I-shaped rails were widely used.
In the early days of rails, there was another important issue, which was whether the rim should be on the inside or the outside. The first thing that needs to be explained is that the wheels of the train are not cylindrical, but conical. The part where the wheel contacts the rail becomes the tread, and the protruding part of the wheel is called the rim. The inner and outer diameters of the tread are not equal. This design is called the “Tapered/Conical Wheel”. for smooth running reasons, the side with the larger diameter of the tread should be the side where the rim is.
Thus, if the rim is on the inside of the wheelset, as shown in the figure below if the wheelset is moved to the least relative to the track, the radius at which the left wheel meets the rail becomes larger and the right side becomes larger and the right side becomes smaller, so the left wheel travels longer in one revolution than the right wheel, and because the left and right wheels are rigidly connected, the rotational speed is always equal, so the wheelset will move to the right by itself, thereby returning to the center of the track, and vice versa, thus ensuring the train safe to run.
The reason why the wheelset is designed now is mainly to facilitate the curve. In the curve, the circumference of the rail on the outside of the curve is longer than the circumference of the internal measurement. In order to avoid the situation that one wheel rotates and one wheel slides, the tread of the wheelset is designed as a conical slope. Strictly speaking, the wheel tread curve of a locomotive is divided into several sections, and the curvature length of each section is different. In addition, the wheel sets have a certain amount of axial movement, and the purpose of designing this movement is to improve the reliability of the locomotive bogie when passing through the curve.
The original rail material was cast iron, which was soon replaced by steel because of its low strength and easy breakage. So, how are rails made?
First, molten steel is cast in molds into long strips of rectangular cross-section, called blooms.
These long strips are then reheated for further shaping. The heated steel passes through a Rolling Mill where the steel is stretched and cut again. Subsequently, the steel is heated again and passed through a series of Rolling Mills again to be gradually processed into the shape of the rail.
So how is the rail processed into the desired shape in the Rolling Mill? The answer is through a series of rollers, called Rollers. If you look closely, you can see that the gap between the two rollers in the figure below (shown by the red circle) is exactly the shape of the rail section.
The rails are then left to cool before passing through more Rollers again. In the following two figures, the Roller in the upper one is responsible for straightening the vertical plane of the rail, and the Roller in the lower one is responsible for straightening the horizontal plane of the rail.
Finally, the rails are cut to size and ultrasonically inspected to ensure there are no defects in them. Each rail has its own number, which can be traced back to which batch of Bloom each rail was produced from because if one rail is defective, all rails made from that batch of Bloom are likely to have similar defects. If a rail is produced from the top section of a Bloom, it is sometimes noted separately, as the steel at the ends of the Bloom is sometimes of lower quality than the steel in the middle.
After these processes, the rails can be laid on the track for the train to run safely, quickly, and smoothly.