Also Milan metro line 1 has four rails.
The official reason was to avoid that eddy currents damage the main rails but unofficially the choice was made to ease an eventual future conversion to rubber tyres use (in the 60s there were strong pressures by some industries to put on use a system called "strada guidata" (driven road)
Alcohol is an enemy. Man that escapes from the enemy is a coward!
That's what I am thinking too. If the middle rail is -260 volts, and the running rails are earthed ,then the running rails are 260 volts positive to the centre rail. Certainly enough to prove fatal.I always thought the refuge hole in the centre of the tracks was a safe place to fall, but not with 260 volts D.C in the middle.