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This concerned the operation of the signalling system.
To understand this better, you should open the Google maps link and turn it 180°. There is still a bus stop in a wide street. So there were 2 tracks and the driver had 2 possibilities.
The first was to let his service end there and perform the necessary maneuvers to get on the other track.
The second is the most obvious: just follow the line and enter the single-track section.
Explaining the whole system would lead me too far, but it was an additional signal that served as a safety device, confirming that a tram had moved onto the single-track section.
This was not activated when a tram was moving in front of or on the intersection. This is related to the system of pre-signals and main signals.
This also applied if a tram passed there, heading for the above stop: the front signal went out, the main signal remained red.