How does the European electric distribution work? I see many sets of delta wires in a corridor, sometimes 15 wires. Why not one big wire per phase instead of doubled wires and multi wires? What voltages do they use?

Also, for trains, the overhead wires have big insulators, indicating high voltgage, but some street cars, it is hard to find insulators and the poles are streel (at first I thought the poles were plastic acting as insulators.) What are the voltages used in these places?