🚀 go-pugleaf

The railway technical pages at www.trainweb.org will give you some answers:

"DC, on the other hand was the preferred option for shorter lines, urban
systems and tramways.  However, it was also used on a number of main line
railway systems, and still is in some parts of continental Europe, for
example.   Apart from only requiring a simple control system for the motors,
the smaller size of urban operations meant that trains were usually lighter
and needed less power. Of course, it needed a heavier transmission medium, a
third rail or a thick wire, to carry the power and it lost a fair amount of
voltage as the distance between supply connections increased.  This was
overcome by placing substations at close intervals - every three or four
kilometres at first, nowadays two or three on a 750 volt system - compared
with every 20 kilometres or so for a 25 kV AC line".

Generally voltages were low on old systems because electrical gear was heavy
and bulky if needed to reduce voltages for the DC motors, designed to
operate at around 600 volts.    1920s London tube stock for example had
about a third of each motor carriage devoted to the control gear, if you had
to reduce voltage from the line supply with the technology then available
you would have lost considerably more revenue space.

Regards

David Bennetts