Track circuit operating clips are used for the system you're talking about, they're just a pretty simple arrangement of giant spring clips with a deliberately short lead between them, workers are trained to apply the clips starting with the side away from the third rail, the lead is too short to make it possible to connect the third rail to the running rail if you've done it this way.
The big metal bar is different, as the poster explained it will short the (supposedly dead) power circuit in a third rail system, taking several hundred volts at quite a lot of amps until the short is detected, hence it can't just be a couple of metal clips and a cheap cable like TCOC. If some idiot re-enables power to the circuit or a fault elsewhere re-energises it despite it notionally being switched off, the bar will turn that into a full short and everybody will know there's a problem, although I'm not sure that would save anybody who happens to actually be touching the now surprisingly live rail at the time it's energised.
On overhead systems there is similarly an arrangement where a worker - after confirming that the power is supposedly dead - ensures this is true by physically grounding it. Again it's a failsafe.
The big metal bar is different, as the poster explained it will short the (supposedly dead) power circuit in a third rail system, taking several hundred volts at quite a lot of amps until the short is detected, hence it can't just be a couple of metal clips and a cheap cable like TCOC. If some idiot re-enables power to the circuit or a fault elsewhere re-energises it despite it notionally being switched off, the bar will turn that into a full short and everybody will know there's a problem, although I'm not sure that would save anybody who happens to actually be touching the now surprisingly live rail at the time it's energised.
On overhead systems there is similarly an arrangement where a worker - after confirming that the power is supposedly dead - ensures this is true by physically grounding it. Again it's a failsafe.