Hi,
Which is the use of the clamps at mechanical boxes?
The Rg symbol
Thanks

At mechanical boxes we have :
Solid numbers ( 1,2,3 ) -> to change tracks
Wk symbols ( Wk1,Wk2...) -> to unlock a locked track
And Rg.... Which is the purpose of that?
Thanks

The levers marked with an Rg symbol followed by a number, are connected to a lock, that locks the switch in a specific position. The switch and it's locking position is also specified on the lever. The purpose of this locks is to make sure, that the switch is in the correct position, and to lock it in that position. When it's used and more information about it can be found here (page in polish)

The rail english equivalent is "Facing point locks".