Switching and locking systems

The moveable parts of turnouts allow interruption-free switching between individual track lines. Safe operation requires that the moving parts of a turnout are properly positioned and locked. The completion of the positioning process must be identifiable without doubt. In the past, turnouts were placed in the desired direction by moving the control lever and then operating the turnout lock so that it was secured in its end position.

Turnout locks must enable trains to run smoothly over the turnout (at speeds of up to 160 km/h[1]) in the network of DB), and prevent them from derailing. This is not possible with movable crossings.

Today, turnouts are mainly mechanically or electrically controlled remotely. The control command is passed on by a signal box by means of chains and wires to the turnout. More modern signal boxes send commands directly to the turnout. The switching is done by a locally installed electrical point machine.

Locking is ensured by the tongue rail being held against the stock rail. This results in there being no space between these rails for the flange of the wheel to enter. The wheelset can, therefore, only go in the specified direction. A one-sided gap between stock rail and tongue rail might result in a possible derailment. While on one side, the tongue rail lies against the stock rail, the locking system on the opposite side ensures the necessary clearance for the wheel. In Austria, Switzerland and Germany external locking systems are generally used. The end position of the turnout is produced by the actuating linkage and associated locking module and this is monitored by the end position checker. Only after the position of the switch has been proved is it possible to release the route for the movement.

Turnout switching device
© Thorsten Schaeffer
Turnout locks
© Thorsten Schaeffer

The actuating linkage can also be operated with hydraulic systems (HYDROSTAR). Locks can either be moved directly via a push rod connected to the adjacent drive or by actuating rods over some distance. As a rule, central locks are connected with the drive via an actuating linkage, but it is also possible to drive each lock individually. Several motors are used to drive the very long tongues of high-speed turnouts. Various resistances work against the switching forces. These are made up of the existing friction forces between the rail foot and slide bearings as well as the bending forces that occur against the deformation of the rail. Treating the slide bearings with oils or greases and the reduction of the resistance torque of the rail by machining the foot of the tongue help to reduce the resistance. Especially in high-speed turnouts passing trains set up vibrations in the long tongues which are also transmitted to the switching and locking devices.

Locks secure the tongue in its position. A distinction is made between hook locks, bracket locks (clip at the top brackets), and latch locks.

Turnout drive of a high-speed turnout
Turnout drive of a high-speed turnout
© Fabian Hansmann

You can find suitable specialist literature about the topic here:

The Basic Principles of Mechanised Track Maintenance

The Basic Principles of Mechanised Track Maintenance

This book is dedicated to the many people involved in the day to day planning and performance of track maintenance activities. Providing a practical approach to everyday challenges in mechanised track maintenance, it is not just intended as a theoretical approach to the track system. 
Railways aim at transporting people and freight safely, rapidly, regularly, comfortably and on time from one place to another. This book is directed to track infrastructure departments contributing to the above objective by ensuring the track infrastructure’s reliability, availability, maintainability and safety – denoted by the acronym RAMS. Regular, effective and affordable track maintenance enable RAMS to be achieved.

Best Practice in Track Maintenance, Vol 1 - Infrastructure Management

Infrastructure Management Volume 1 looks at aspects of infrastructure management with particular reference to the single European railway area. Based on best-practice examples from Central Europe, measures for the targeted retrofitting and improvement of the infrastructure maintenance of the existing network are presented. In many cases, infrastructure operators are faced with a generational change, which accelerates the process. Modern information and communication technology can simplify the comprehension and presentation of complex contexts. Modified approaches to asset management and life-cycle management enable implementation of the "transparent permanent way" or the "railway 4.0".

  1. [1] Fendrich, L.; Fengler, W.: Handbuch Eisenbahninfrastruktur. Springer, Berlin, 2013.