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 Heruntersignalisierung mit Hl-Signalen 
Signal 2 says clear, expect clear
Signal 4 says clear with maximum speed, expect 40 or 60 km/h at next signal
Signal 6 says clear with 40 km/h, expect stop
Signal 8 says stop
   
Ks System Heruntersignalisierung mit Ks-Signalen 
Signal 2 says clear, expect clear
Signal 4 says clear with maximum speed, expect 60 km/h at next signal
Signal 6 says clear with 60 km/h, expect next signal at 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 8 says stop
   
Ks System
at clear
Heruntersignalisierung mit Ks-Signalen 
All signals say clear, expect clear
Signal 6 since that signal requires no (further) speed reduction, the white light is off
   
Sv System Heruntersignalisierung mit Sv-Signalen 
Signal 2 clear, expect clear
Signal 4 clear, expect medium speed
Signal 6 medium speed, expect stop at reduced distance
Signal 8 stop