Locomotives are a better option for trains of 4 - 6 cars over longer distances with fewer stops, which is the current long haul set up in Victoria. Eg Albury
Duncs, I'm interested in the reasons for this? Is it better fuel consumption? Interesting that in the UK they've eliminated loco-hauled trains on the West Coast Main Line and many cross country routes, in favour of the Voyager, which is a 4 or 5-car DMU (depending on configuration). And Voyagers run routes as long as 5 or more hours (e.g. Penzance to Birmingham; Birmingham to Aberdeen).
I suspect that in the UK they went to the Voyager as an upgrade from the Inter City 125. You are looking at 200kph trains versus 115 kph trains here. Also most of the locomotives were from the 1970's and were wearing out. Even so there is the type 67 locomotive (made by EMD) in the UK that does 200kph. It does mail, freight and some passenger work.
Incidentally in Ireland there is the EMD 201 series (3200hp) that does 160kph and pulls passenger trains from Dublin to Cork (the equivalent of Melbourne to Warrnambool) in a 2 hours 40 minutes express service.
Recently the Irish Railways introduced new carriages for this service to improve comfort and frequency. They did buy some new DMU's as well but will use them for the shorter stop - start routes.
Voyagers replaced some HSTs but also 1960's locos and stock (class 47 and Mk2 coaches). The displaced HSTs are still going, have been leased instead by GNER and FGW for the East Coast and Western Mainlines.
The WCML went over to Pendolino electric units, replacing electric locos (87s from the 70s scrapped, 90s from the er..90s, cascaded to 'one' for the Anglian services London-Norwich).
The Midland Mainline dropped some HSTs for 'Meridian' an improved variant of the Voyager.
The ECML (London KX to Yorkshire/North East/ Scotland) is still loco hauled - class 91 electrics with Mk4 stock, and HSTs with Mk3 stock.
The western routes out of London to the West, South West and South Wales are still HSTs being re-engined with MTUs.
Virgin, who introduced Pendolino and Voyager units on those long hauls, have come in for some flack. Although passenger feedback has been favourable, and the new fleet has allowed increased frequencies that have drawn in new passengers they are expensive and inefficient units and are less environmentally friendly than there predecessors.
67s were designed by EMD but built under contract in Spain. They were specifically for mail services, which all but evaporated a couple of years after they were introduced
They still have a couple of mail contracts, charter trains (no booked passenger trains), her Maj.'s personal train, and a couple of lightweight freights (general feedback on UK forums being that they could not pull the skin off a rice puddin'
Units are 1) generally cheaper 2) offer operational convenience 3) tend to accelerate away quicker.