Most Danish double tracked lines are equipped with line Block Systems type DSB 1954 or an equivalent type of Block System. Below is the signal layout of a typical double tracked line with Line Block System type DSB 1954. The line is equipped for right hand running as the normal direction. Simple facilities are provided for Wrong Main Running (Danish: Venstresporkørsel).
The line starts at the station Exit Signals (Danish: Udkørselssignaler). Between the Exit Signal and the Entry Signal (Danish: Indkørselssignal) of the next station is a number of Block Sections (Danish: Blokafsnit), in this example 4. Each block section is protected by an Automatic Block Signal (Danish: Automatisk Mellembloksignal). The Exit Signal protects the first block section, and is in reality an ordinary Automatic Block Signal being held at "Stop" by the station interlocking until needed to clear for a train. Intermediate Block Signals on most such lines are approach lit, i.e. their light is switched off until a train approaches. Contrary to, say, North American practices, the Danish Block Signals never switch off the "Stop" aspect and instead of actually switching off the green lamps they are instead dimmed. This way the Block System is still able to tell whether the lamp has failed or not. The dimmed green light is visible at night in dark locations. Signals thus showing dimmed "Proceed" or "Proceed Through" are sometimes referred to as showing Dark Green (Danish: Mørkegrøn). Dimmed signals on the diagrams on this page are shown as displaying a darker green light.
Running Wrong Main the line is regarded as one Block Section. Wrong Main Exit Signals (Danish: Venstrespor-Udkørselssignaler) are placed adjacent to the ordinary Exit Signals. At the next station are Wrong Main Entry Signals (Danish: Venstrespor-Indkørselssignaler), acting as simplified Entry Signals for trains approaching Wrong Main. Wrong Main Entry Signals are no longer being installed as ordinary Entry Signals are being used instead, but a large number are still in service.
As shown above Exit Signals are normally held at "Stop". When an Exit Route is locked in the station, the Exit Signal is released to perform as an ordinary Block Signal, except that Exit Signals are constantly lit. Shown below is Exit Signal behavior when an Exit Route is set.
When the train passes the Exit Signal, the Exit Route is released. Entering the first Block Section also lights up the first intermediate Block Signal.
As the train moves along the line the Block Signals change to "Stop", then to "Proceed" and eventually to "Proceed Through". Since no train is following close behind in the example below, the signals only shows Dark Green.
In our example the train approaches the second last Block Signal showing only "Proceed", due to the following station Entry Signal being at "Stop". The second last Block Signal only change to "Proceed Through" if the Entry Signal changes to "Proceed" or "Proceed Through" for a route allowing high speed (75 km/h, 45 mph, or more).
When the train has passed the Entry Signal the block system is back in its normal state.
Due to technical failures a Block Signal sometimes stays at "Stop" even though the train behing the signal has entered the following block. In this situation the line may be released for "Stop and Proceed", meaning that all Block Signals showing red to an apparently free block section switches to "Stop and Proceed". The release is valid for the whole line at a time. The Exit Signal stays at "Stop" unless an Exit Route is set.
The basic condition for allowing a train to run Wrong Main is the Block System on the line being in its normal state. In the example below a train is waiting to be signaled to the Wrong Main. On many older stations only a few Wrong Main Exit Routes are implemented as they are rarely used. Typically the routes are to and from the passing siding.
Setting an Exit Route towards the Wrong Main Exit Signal switches the line to Wrong Main Running. First the Wrong Main Exit Signal, all intermediate Block Signals and the Wrong Main Entry Signal on the next station switches to "Stop".
When the line is checked to be clear, the Wrong Main Exit Signal changes to "Proceed". The Platform Exit Signal only shows "Proceed" for trains to be running Wrong Main. Speed in the Wrong Main Exit Route is restricted to 40 km/h (25 mph). Speed on the line is restricted to 80 km/h (50 mph).
The Wrong Main Exit Signal changes to "Stop" when the train occupies the line.
Any clear Wrong Main Entry Route has the Wrong Main Entry Signal showing "Proceed", allowing the train to enter the station at 40 km/h.
When the train passes the Wrong Main Entry Signal the line restores its default state.