AC does better power delivery on long distance
Nooope. High Voltage Direct Current is in fact technically superior when it comes to point-to-point connections as you can use smaller conductors (no skin effect means the current travels through much more of the cross-section of the conductor) and you only need 1-2 cables/wires vs 3 for 3 phase AC.
High Voltage Alternating Current is used universally instead of HVDC because it's much, much easier to step up and step down the voltages with transformers. Only modern semiconductor power electronics have gotten DC-DC converters even remotely efficient enough to compete with AC. AC is also easier to generate with rotating plant (i.e power stations) as synchronous generators don't need brushes; AC circuit breakers are easier to build because you have zero-voltage points in the AC waveform that naturally interrupt the arc. AC power systems are also simpler to control because you can use frequency regulation to do so - power demand increases -> system frequency decreases -> raise power output and vice-versa.
AC and DC are "not dual voltage". Different technology - Nikola Tesla vs Thomas Edison.
1500v DC or 25000v DC - that is "dual voltage"
is the British, French and essentially the global greenfields electrification standard, not DC.
so they [HCMT] will be restricted on the routes they can run?
Technically, no. The thing with DC electrification is that the voltage isn't a constant 1500V everywhere - it changes with distance to the traction substation, train current draw and so on. All trains that work on 1500VDC can deal with at least a couple hundred volts variation, just like consumer appliances do. The HCMT specification simply calls for a higher voltage tolerance than usual - and a 3000VDC capability.
Now, when you start talking about dual/tri/quad voltage capability in locos and EMUs, there's going to be a mix of AC and DC specifications usually. Most of the Euro stuff is 1500VDC/25kVAC or 3000VDC/25kVAC for instance, but 15kVAC (@ 16.7Hz) is the incumbent standard in the Germanic countries so it's often chucked in as well on multi-voltage locomotives as they can afford the weight penalty of an additional transformer.
So the HCMT can handle 1500VDC with restricted power draw, as a lower line voltage at the same power draw = higher current draw = higher risk of overloading the traction substation equipment. The new/replacement substations installed as part of CTD (Skyrail) are both capable of operating at a higher voltage (1950VDC, possibly 3000VDC but don't quote me on that one) and also have a much higher current capacity in concert with the beefier contact wires installed with the new overhead staunchions that are being put in all through the Caulfield to Dandenong section.
This has been bought on by two things:
- HCMTs draw more power than the existing Metro fleet for a variety of reasons (stronger aircon, better acceleration etc). I think they also might be heavier than X'Traps and Siemens units as well due to the new crashworthiness standards that have been imposed in recent times and the higher crush loading capacity (more people = more weight, also stronger carriages).
- The Metro timetable is becoming more intensive, placing a higher continuous power load on the existing traction substations throughout the network.