Train sets and locomotives
State-of-the-art high-speed (HS) trains don’t follow the traditional rules of the rails. Older trains still use one or two locomotives that contain all the traction motors and locomotives are positioned in the front as a pair, or the front and rear to better distribute the tractive forces.
Modern HS trains, however, put traction motors in almost every car for better force distribution and a more-comfortable ride. Distributing power over more axles also lets trains accelerate and decelerate faster. That’s because the powered wheels rely on friction between the wheels and rail to transmit power. Sending all the power to only two or three axles, especially when starting out from a dead stop, increases the likelihood that the power would overcome the friction and spin the wheels. There are also conditions, even with distributed power, when there’s not enough friction to get a large train rolling or up an incline. Ice or wet leaves, for example, can severely limit the frictional force and cause wheels to slip. To overcome this, HS trains carry dry sand, just like their slower-speed cousins. It gets dropped in front of powered wheels to increase frictional forces.
Spreading the tractive force between axles, mounting the traction motors below the floors, and eliminating the locomotive can give modern HS trains 20% more space for passengers in the same-length trains, according to Siemens.