So why knock the train wheels? Probably everyone has asked themselves this question. How can round and smooth the wheels banging on the rail absolutely flat canvas? The answer lies in the structure of the railway rail.The railway line was never perfectly smooth. Modern technology is still unable to produce and the right to lay many kilometers of rail, and even considering all the turns and the location of the arrows. Railway road is a plurality of individual rail segments, stacked back to back. And the reason here not only in the complexity of manufacture and delivery of solid rail necessary length. As you know, the body expands when heated. This happens with railway rails regardless of passenger or freight train will ride over them, tram, train or metro train. Knock allows the metal to freely expand in length. It is in these joints, due to the current of the hump and there is a knock. Logically, we can assume that the summer wheels are knock less. However, even the slightest gap between the rails enough to hear sound.The second reason that can knock the wheels – the appearance of the slider due to the SW from the wheelset. That is, the wheel for some reason just stuck. However, modern rail transport is established a special electronic control system, and completely eliminates this phenomenon.It should be noted that it is now increasingly used a special system of laying the rails without joints. This technology significantly reduces energy losses and wear of wheel pairs of the train and the rails. It also reduces the number of defects that appear in the material of the rail at the joints upon impact of the wheels. Often this innovation is used on the tramways and the metro.