Service connecting branches of 1/2 Lines
It is a group of several tunnels, built as part of the First line of the Leningrad metro. In general, it started with them.
After 1961 and 1963, half of the branches became unnecessary, and the rails in them were dismantled. The second half today often carries trains with passengers. For example, their exit ramps:
At the turn of the century, for a long time it was the norm that in the morning and evening rush hour trains passed here between lines directly with passengers. It was convenient to change the line without changing the train.
The station Technological Institute-2 platform is clearly visible from here:
Label: station border Technological Institute.
The connecting branches themselves are very short, about 100 meters. On the other hand, an inactive tunnel with rails dismantled in 1961 is used for service purposes:
From it you can see a piece of the station Technological Institute-1 hall:
On the stage Technological Institute-2 - Sennaya Ploshchad there is a disassembled track that worked in 1961-1963 when the station was final on the 2nd Blue Line. In the opposite direction, in the middle of the stage, there is a former path to the 1st Red Line. There is lighting and even rails, but they do not intersect with the main tracks. Now they have sheds for storing track service equipment and signaling equipment.
Thus, when leaving for the station Frunzenskaya via 2nd Blue Line you can see in the tunnels trains of 1st Red Line: first, from the left through the unused tunnel - train arriving from the station Baltiyskaya, and then on the right through the working tunnel - a train descending towards it.
Track diagram taken on metro.vpeterburge.ru.