If a manually controlled crossing monitored by CCTV sufferers from a failure of the CCTV system (rare, because two cameras and two monitors are provided), all that is required, is a level crossing attendant.
The level crossing attendant just needs to keep the signaller informed when the crossing is free of road traffic, then once the barriers have lowered, confirm that the crossing is clear.
Alternatively, the level crossing attendant can take local control (a local control box) of the crossing.
Normally automatic half barrier crossings don’t have CCTV. The signaller just gets indications that either show the crossing is in a normal / operational state, or is in a failed state. A local power failure will cause the crossing to show failed (that’s IIRC), but the crossing should continue to work on the battery supply (at least for some hours).
With a manually controlled crossing monitored by CCTV, a local loss of mains power will result in the crossing illumination flood lights failing. The rest of the crossing equipment may not be affected if powered from the signalling 650V supply. Or if the battery charger is fed from the local mains supply, the signaller will get a ‘power failure’ alarm. The crossing should continue to work on the battery supply (at least for some hours).
However, during the hours of darkness, without illumination from the flood lights, the CCTV picture will not be good enough for the signaller to see if the crossing is clear…