Train is one of the most important means of transportation for human beings. With its high speed, safety and economy, it provides convenience for people’s travel and the transportation of goods.

However, have you ever paid attention to the running gear of the train, that is, the part connecting the wheels and the body? Do you know why there are four wheels on a train axle instead of two? What is the function of the two small wheels in the middle?

First, we need to understand the structure of the wheels and axles of the train. The wheels and axles of the train are a whole, called a wheelset. The outer edges of the two wheels of the wheelset are conical, called treads, which are in contact with the rails and bear the weight and traction of the train. The middle part of the wheelset is a cylindrical shaft, which is connected to the bogie of the body and reduces friction through bearings. There are also two small brake discs at both ends of the wheelset, which cooperate with the brake calipers on the bogie to achieve the braking of the train.

So, why are there four wheels on a train axle? This is mainly to improve the braking effect and safety of the train. The braking system of a train is generally divided into three types, namely disc brakes, brake shoe brakes and electromagnetic track brakes. Disc brakes are brake discs installed on the axle of the wheelset, and the brake caliper is used to press the side of the brake disc, and the kinetic energy is converted into heat energy through friction, thereby slowing down the train. The advantages of this braking method are smoothness, no noise, and a large braking area, which is suitable for high-speed trains. Brake shoe brakes are brake pads installed on the tread of the wheelset, and the brake caliper is used to press the contact surface between the brake pad and the tread, and the wheel is stopped by friction.

The advantages of this braking method are simplicity and low cost, which is suitable for low-speed trains. Electromagnetic track brakes are a row of electromagnets similar to teeth installed on the bottom of the car. When braking, they are pressed down against the rails, and the car is stopped by electromagnetic force and friction. The advantages of this braking method are strong and fast, and it is suitable for emergency braking. From the above introduction, it can be seen that disc brakes are the most commonly used braking method, and the brake discs are the two small wheels on the wheelset. Without these two small wheels, disc brakes cannot be achieved, and can only rely on brake shoe brakes or electromagnetic track brakes, which will reduce the braking effect and safety of the train.

In addition to the braking effect and safety, the four wheels on the wheelset also help improve the dynamic performance of the train, that is, the train’s driving performance on curves, slopes and uneven tracks. The wheelset of a train is a rigid whole, and it cannot change direction when turning like a car. So, how does the train run on a curve? This requires the use of the tapered structure of the tread of the wheelset. Because the tread of the wheelset is tapered, its inner and outer diameters are different, that is, its inner and outer circumferences are different. When the train runs on a curve, the radius of the inner and outer rails is also different, that is, their arc lengths are different. In order to keep the two wheels rotating synchronously, the circumference of the inner and outer wheels must be equal to the arc length of the inner and outer rails, which means that the speeds of the inner and outer wheels are different, the inner wheel rotates slowly, and the outer wheel rotates quickly.

In this way, the train can run smoothly on the curve. However, this method also has a disadvantage, which is that it will produce a certain amount of slip, that is, the actual movement trajectory of the wheel does not completely coincide with the ideal trajectory of the rail, which will cause friction and wear between the wheel and the rail. In order to reduce this slip, it is necessary to maintain a certain lateral clearance between the wheels and the rails, that is, the width of the wheels should be less than the spacing of the rails. In this way, the wheels can move freely between the rails and adjust their positions according to the radius of the curve, thereby reducing slip.

This is why there are two small brake discs on the wheelset of the train. They can not only achieve disc braking, but also serve as lateral support for the wheels to prevent the wheels from moving excessively between the rails and causing the risk of derailment. From the above, we can draw the following conclusions: There are four wheels on a train axle to improve the braking effect and safety of the train, as well as to improve the dynamic performance of the train. The two small wheels in the middle are brake discs, which cooperate with the brake calipers to achieve disc braking, which is the most commonly used braking method.

The treads of the wheelset are conical, they are in contact with the rails, and the characteristics of different inner and outer diameters are used to achieve smooth travel of the train on the curve. The brake disc can also serve as a lateral support for the wheels to prevent the wheels from moving excessively between the rails and causing the risk of derailment.