Variable Speed Signalling

With the old Hp system, block signals were either at clear or stop. With dense traffic, that could mean when a following train goes faster than a preceding one, the following train will eventually see a distant signal at caution, indicating the next block signal being at stop.

The train then has to brake and probably stop for a few minutes. A more economic and comfortable would be to signal a lower speed at the signal in rear, so that the following train will reach the signal in advance at clear. However this practice has been in use on urban railway (S-Bahn) lines only. In Berlin DR's Hl signals and the new Ks signals are used, while Hamburg uses Sv signals.

In the example below, the distance between signals 6 and 8 (the one at stop) is shorter than the braking distance. With both signalling systems, the preceding signal 4 induces a safe speed at signal 6. With the Ks system, the shorter distance is also indicated by a white light, see below.

Hl System

	Signal 8	Stop.
	Signal 6	Clear with 40 km/h, expect stop.
	Signal 4	Clear with maximum speed, expect 40 or 60 km/h at next signal.
	Signal 2	Clear.

Ks System

	Signal 8	Stop.
	Signal 6	Clear with 60 km/h, expect stop. Since the next signal is at shorter distance and requires (further) speed reduction (in this case stop) the white light indicating the shorter distance is on.
	Signal 4	Clear with maximum speed, expect 60 km/h at next signal.
	Signal 2	Clear, expect clear.

Ks System
at clear

	Signal 8	Clear, expect clear.
	Signal 6	Clear, expect clear. Since that signal requires no (further) speed reduction, the white light is off.
	Signal 4	Clear, expect clear.
	Signal 2	Clear, expect clear.

Sv System

	Signal 8	Stop.
	Signal 6	Clear with medium speed (40 km/h), expect stop.
	Signal 4	Clear, expect medium speed (40 km/h).
	Signal 2	Clear, expect clear.