550-750 V DC are fairly common for urban systems where lines are relatively short and in the case of 3rd rail systems voltage drop is limited by the huge cross-section. Trams also need to be lightweight, installing a huge transformer isn't really an option. So low-voltage DC was a fairly natural choice, often directly fed to motors with series resistors and load resistors for generative braking. Only in the late 1970s solid-state motor controls were introduced but by then 550-750 V DC had already been well-established as the standard voltage for tram, underground and metro systems. These voltages are also less of a safety concern than 10+ kV and require much smaller insulators, keeping appearances in city streets tidier.
Why the DART uses 1.5 kV DC is beyond me, I think at that time there was already an international agreement to use 25 kV/50 Hz for railways unless there's a legacy system already in use in that country.
Austria is generally 15 kV/16.7 Hz for railways (introduced around 1900 for the Vienna - Bratislava interurban railway that ran as a tram in both cities and as a proper railway outside, changing locomotives from DC to AC in the outskirts of each city) but there are different systems - 650 V DC for most trams, 750 V DC for the Vienna Underground (both 3rd rail bottom contact and overhead wire), 850 V DC for the local railway Vienna - Baden (WLB) running with beefed-up Duewag tram cars and Bombardier LRVs and a few other DC systems for older electrified regional train lines. All new electrification projects are 15 kV/16.7 Hz except trams and underground systems.
Since all new rail vehicles use VFDs to drive the motors the frequency doesn't really matter anymore. Either the current is rectified at the substation and converted to variable-frequency AC inside the locomotive/motor car or it's rectified and converted to AC inside the loco. AC has the huge advantage of being able to use transformers and much higher voltages, limiting current and voltage drop. DC mainline railways use scarily huge overhead wires!
