"PoSA" style signalling on LO (East London Lines)

There are quite a few of them on the East London Line. They are on key signals, such as terminus platform starting signals and signals protecting junctions, on EL prefixed signals (controlled by East London Line SB).

They are used, however it is very rare for us to get signal failures, which is why you probably thought they're not used. I have only seen one in use in my six years here and that was about four years ago. For those not aware, if a signal can not be cleared for whatever reason, yet the signaller can still set a route, then they can activate the PoSA as a more efficient way of authorising trains to proceed in a high-frequency situation. The trains are still only authorised to proceed at caution up to the next signal.