If a vehicle's drive stops working, it is not possible to move forward. This may be annoying, but usually not dangerous. If the brakes are faulty, on the other hand, all the alarm bells need to ring straightaway, since to a certain extent, the brakes are the most important part of any vehicle. This is particularly true for trains, which may carry many hundreds of passengers.
From handbrakes to modern stop technology
At the beginning of the 19th century, the first railways were braked by hand and for each car. Over time, centrally controlled braking systems became established, operating over the entire length of the train. Back then vacuum brakes were common, while compressed air is primarily used today.
The two braking processes are in principle very similar: brake pistons on the wheel axles are actuated by brake cylinders. The cylinders are connected to the control unit by lines leading from the traction unit to the end of the train. The train driver actuates a valve to initiate braking. In the case of compressed air brakes, overpressure is led into the brake lines, which comes from a compressor and supplies the energy for the braking process. In the case of vacuum or suction air brakes, the valve serves to reduce an existing negative pressure. It keeps the brake pistons away from the axles during travel. When the valve is actuated, the incoming atmospheric pressure can apply pressure and initiate braking.
Advantages of vacuum brakes
Compressed air brakes do have a disadvantage in terms of the design: they need to compress ambient air for the braking process. During cold weather, the moisture in the air can condense. This leads to a drop in pressure and thus to a reduction in braking performance. If the weather is really icy, the condensation can freeze and clog the brake lines.
This danger does not exist with vacuum brakes. To make them ready for operation, the vacuum pump builds up vacuum in the brake lines after switching on the control unit. This prevents moisture from entering the brake system from the outside. A pressure differential of about 690 millibar between the brake system and atmospheric pressure keeps the brakes in the release position. Braking is triggered when the train driver opens the valve. This also happens automatically in the event of a system fault.
This ensures operational safety, and not only at sub-zero temperatures. In addition, vacuum brake systems can be easily regulated. They retain their full effectiveness even after repeated actuation in quick succession – important advantages for operation on long downhill stretches and when manoeuvering. Vacuum braking systems are therefore still in use today, for example on many narrow-gauge and small trains, diesel locomotives for manoeuvering operations or tunnel construction, mountain trains in Switzerland and Austria as well as in trains from the South African and Indian railway networks.
Braking a long monster of a locomotive and its attached cars is a complex technical challenge. In view of the accelerated mass, long braking distances must be taken into account. Depending on the train length, between 5 and 25 tons per wheel set have to be stopped (for comparison: for a passenger car it is an average of 0.7 tons per wheel set). The wheel set brakes, such as the compressed air and vacuum brakes, are the most important, but still only one component. The engine brake turns the electric motor of the locomotive into a generator and thus generates electricity in addition to the braking effect. In emergency braking, rail brakes are used: brake shoes are lowered under the chassis and pulled onto the rails by integrated electromagnets.
State-of-the-art brake technology is found in eddy current brakes. Their effect is based on electromagnetic scattering in the rails. The braking force is generated contact-free and precisely dosed between the brake and the rail head. However, it depends on the speed, which is why this technology is only used as a supplement.
When braking trains, it is also important to bear in mind that if the braking force is uneven, compressions and strains can occur over the length of the train. This happens, for example, with compressed air brakes because the overpressure of the compressor arrives in the rear car only delayed due to the long air line. These then press against the already braked part of the train. If, on the other hand, the rear part of the train were braked earlier or longer than the front part, there would be a danger that the train would tear apart longitudinally. In order to prevent such effects, the application and release times of individual vehicle brakes are adapted to the travel speed and the desired braking effect with the aid of changeover devices.