There are several main signals that can operate automatically. We will give an overview:

1) Main signals with a crown lamp
2) Main signals and tunnels
3) Intermitterende signals in more complex junctions
4) Simplified signals that open automaticly

Main signals that operate with a crown lamp

Normally all switches are protected by main signals, but there are a few situation where the traffic over the switch is very low. This main signal of the main line can be outfitted with an extra lamp, the crown lamp.

Picture: 41.1

The crown lamp is only illuminated when the signal works as a permissive signal and the signal shows red. This lamp allows the trainengineer to trespass a red sign without a written permission of the signalmaster.
When there are shuntings on this track, the signalmaster will force this signal to red. The crown won't illuminated, and the trains on the main line will be stopped.
See also the chapter permission on the Disturbance page

An example

Just outside the station of Tongeren a line branches off to Hessenatie car terminal.

<< station Tongeren                                                                                                             station Glons>>

A train proceeds in the direction of Tongeren. The entry signal of Tongeren is cleared.

The crown is illuminated.

The crown lamp stays illuminated until the train exits the block and then it will be back in the default state.

When a train shunts in the siding the main track is blocked

In the block that the signal (with the crown lamp) protects, there can't be a level crossing. The junction at Beveren Military depot is situated in this circumstance. Left Sint-Niklaas right Beveren. The main signals working as a permissive signals don't have a crown lamp.

You don't have to follow always the rule. The switch at Herentals West is a nice example
up-left: direction Nijlen   up-right: direction Herentals
left: marshalling yard  right: old branch line to Aarschot

Picture: 41.5

The switch on the left is protected with permissive signals. The signals carry a cross crown. (See enlarged picture)  Even when they show red the train may continue. The main signal of the marshalling yard can only show double yellow even if the next signal is open.

Picture: 41.6

Main signals and tunnels

The Belgian Veurs tunnel is up-hill protected with a main signal. This signal (recognize the telephone-box) will work mostly as a automatic signal. The difference with a normal automatic signal is the fact that a train will not enter this tunnel if the track is occupied (no crown = no entry).

Picture: 41.7
© Jan Uyttebroeck

Intermitterende signals in more complex junctions

Picture: 41.8
©ferrosteph

Main signals that signal frequently the same route, can work as permissive signals. It are mostly the cross junctions between two stations. The two main signal of the normal track can work as permissive signals because the most trains are signaled straight through. So, this means that most trains run from one station to an other on the same track. In this case the crown lamp is lit all the time. Only when one track is obstructed the signals work indeed as main signals.

Here, the main signals of the middle cross junctions work as permissive signals. Notice that only the normal track signal are provided with the crown lamp. 

A train proceed from left to the right, and occupies the first block after the cross junction. The signal is closed.

But,,, with the new computer controlled signals this principle is seldom used nowadays. I must confirm that I only saw it once working. Mostly the main signals stay closed until needed to open for a train. The computer can 'see' the train when the train gets into the block with the yellow signal. Then the main signal is opened automatically by the computer (if the track is free of course).

Nowadays with computer controlled signalling

Simplified signals that open automatically

Simplified signals for level crossing
Some simplified signal opens automatically when a train approaches. They are used on tracks with restricted speed.

Picture: 41.9

When the train approaches the level crossing, both signals open

Simplified signals that clear the track
These signals are used on shunting tracks where the visibility is poor. When a train arrives, the signal verifies if the track isn't occupied before it opens. On a telephone box there is an indication of the occupied track.

Picture: 41.10