The truth is.... it's a myth.
I've known a few drivers who've had 3 fatalities (one of whom was only a couple of years off retirement at his last one). I've known a couple of drivers who've had 4 fatalities, and one who'd had 5.
All of them came back to work after varying amounts of time off.
There used to be a Criminal Compensation payment (usually amounting to about £1000) that was sometimes paid. This barely compensated for lost overtime, nevermind the stress and upset caused to the Driver and their family - who'd had to put up with their loved one waking up with nightmares, and being moody or short tempered - then to go through it all again when the Coroner's Court got in touch.
The payment was stopped a few years ago by the government, who said that suicide wasn't a crime so the payments should stop. This is despite trespass on the railway being a crime, so a crime has to take place for someone to be in a position to commit suicide on the railway?!!
The myth stems from the film "3 and out" mentioned above, where a tube driver has had two and his mates say to him "you just need another one and then you'll be set for life" and that there was a little-known rule about getting either retirement money or so many years pay - a big pay out, after a third fatality.
The comedy is that he befriends a suicidal man and they make a pact to help each other out.
Only the myth gets found out to be a myth, and that the tube driver's mates were having him on.
There has always been black comedy, which is fair enough, but the worst thing about it was the placement of adverts for the film on station platforms, including near where the driver's cabs stopped. Most insensitive.