The oldest trains we have at my job were built by Rohr Industries (they do aerospace now) in Winder, GA in the 70's, those all use standard fasteners as did a lot of American stuff from back then, so I'm very used to 3/8", 7/16" and so on. However metric is so much easier. And also, 00 AWG isn't the biggest we use at work, we actually have 0000 AWG (labeled as 4/0 AWG) wires that carry the third rail power from the collector shoes to the rest of the train. Those wires can handle 1000 volts, 1000 amps without breaking a sweat (that's a lot of juice, we run 900 amp fuses on the train).