Suspect the DC = short distance thing came from streetcars and similar trolleys running at 600-750 VDC. They do need frequent substations feeding them, but that's because of the relatively low voltage and therefore high current and power loss in the wire. Many interurbans pushed the DC voltage up to 1200-1500, which cuts the current in 1/2 from running at 600-750 and allows substations to be farther apart. Boosting to 3KV cuts current (for similar power draw) in 1/2 again. Main problem in the olde dayze was converting between DC voltages - motor generator sets were common, but those have limits. More modern 750-1000 KV DC transmission lines like the Bonneville/SoCal intertie use non-mechanical methods of changing voltages and moving from DC to AC and vice versa.

AC's big advantage was and still is the ability to use transformers for voltage changes. Transformers are relatively simple devices without moving parts. Until high-current rectifiers that would fit in a locomotive or passenger car became available, it wasn't practical to use it for traction power, though. Modern control systems with IGBTs and computerized controls, of course, make using AC traction power easy (if you can find space for the transformer), so modern electrifications are usually at utility transmission voltage and frequency AC (25-50 KV, 50-60 hz). Interestingly, though, the NEC is still 25 hz (11KV or higher) as a holdover from the Pennsylvania RR days.