Basic electrics. For a given power level, higher voltage requires lower current. That's why we have high voltage power lines. Current is what governs the needed wire gage, the number of substations needed, and a host of other things - and it's what produces heat and voltage drop over a length of wire. It's been known almost forever that the highest voltage you can get away with is the best. Work the variations on Ohm's Law. There are of course other issues with high voltage - insulators and air gaps need to be bigger, wires develop a skin effect where the body of the conductor becomes dead weight, and so forth. So as with all engineering it's a balancing act depending on what your goal is in a particular case.

In the olde dayze 1200V was a natural evolution of the traditional 600V trolley voltage. Double the voltage, half the current. And if you ran 1200V in the country and 600V in the city, you automatically got the cars running at half speed in town.

Interesting that Back East the old 3rd rail lines (Long Island RR, Metro North) still use it on essentially open right of way. Yes, there's fencing, but it isn't very consistent. There are public grade crossings. Occasionally, people and animals are injured or even killed. But the cost of truly sealing the r/w is prohibitive so things continue more or less as they are. Little of the LIRR, for instance, could meet the CaPUC's requirements for 3rd rail use.