All other things equal, higher voltage requires lower current (amps) for the same power. Ohm's Law. Power loss (heat generation) in the trolley wire is proportional to the current, not the voltage.

LRT usually requires more power than streetcars and trolley buses. It typically uses 2-4 motors per car and 2-4 cars in a typical train, with larger, more powerful motors, compared to one motor in a trolley bus or 2 in a typical modern streetcar (yes, I know, PCCs had 4; they aren't built that way any more). So most LRT systems use a somewhat higher line voltage than streetcars - typically 750-1000V. 600V requires more frequent substations and heavier wire to supply the line, or equipment that can tolerate a large swing in available voltage (many older streetcars, pre-computers, could still work, poorly, at 300V which at times the voltage sagged to in SF, downtown, pre-BART).

There's a practical limit, obviously, based on cost and safety especially on transit lines: DC systems seldom exceed 1500V, and AC systems might, rarely, use 3000. Also, streetcars *can* be set up to run on 750V; there's nothing fundamental that requires 600 with newer equipment and most historic cars can be modified to work at 750. It's a balancing act with the cost, and it's often cheaper (and provides a bit more fault tolerance) to use a few more packaged substations than to get fancy and expensive with the overhead for relatively light duty. Heavy railroad electrics, of course, need much higher power and typically use (these days) AC transmission at high voltages (25KV as used in the Amtrak NEC these days is low end) - again, reducing the amps needed to produce the power. This is talking only about the trolley wire voltage; internal voltages and AC/DC operation in the equipment can be almost anything with modern control and power conversion systems.