Who sets the railroad course in the right direction?

Hello in this round to comment on the previous answers is too much effort. I’ll put it together. Small secondary and intustrie veins still use local switches. The locomotive driver or Ranger puts the switches himself. Often the locomotives are remotely controllable, like a model car or something. The locomotive driver can thus also sit at the front on a Wagon and does not always have to run along the entire train to set a switch.
Come on. Control unit (described in Jonny’s answer) for use, a control unit takes over the operation of the switches and signals. In order to prevent fatal errors, only combinations which cannot lead to collisions are possible.
To disregard a signal, a locomotive driver error remains. However, secondary tracks that lead into a track are secured by protective tracks. Certainly, a number of people have already wondered what a track with a Prellbock at the end is only a few meters long (more short) in which not even a large locomotive fits. The associated switch secures the scoop when a closing ring is passed over. The train cannot accidentally enter the route.
Once a further section or route is to be traveled, modern computer-assisted technology is used today.
Everything is designed twice and programming ensures faultless operation. The service manager(s) operating in the control unit do not set any individual switches but so-called. Roads. This means that the train is conducted from the entry signal to the target with a single command. The input is effected numerically by means of a computer keyboard. The computer in the background first checks whether a further road bie already touches or crosses planned journey. If both are not the case, he releases the road, repels all the paths for the road and all the hostile paths. Letzes means that no other train ride is possible in the ride. The counter-sample now takes place whether all the switches involved have taken your correct position. Only then is the signal release for the incoming or passing train. Only when the station has reached its destination. If the computer clears the road again. Small stations usually turn on a standard route through the railway station, which can also be controlled in the automated block system of the run. (pass monitoring and signals).
If there may be more knowledge, look at Wikipedia under Stellwerke. Here, quite comprehensively all older and today common types of control units are described.
Goodness to you!