Both and all the time .
Like we keep saying these trains have to be powered to climb the ruleing grades on the north south and east west and they all have 1:33 or 1:40 grades for some part . This means having more power than is needed a lot of the time and using it means burning more fuel dollars .
Often we depart Sydney going south and follow various tin cans so the pace is pretty slow , we are encouraged to put as many units off line as possible and run one or two flat out . Average speed is about the same but the fuel burn rate isn't . If the trailing units aren't NRs they can't be put off line or their power reduced on the move and its not always possible to stop go walkies and fiddle with engine control switches .
To even up fuel levels between units means working some harder than others and the only way to do that is to have control of each one from the front .
Think of it like this , Intermodal trains are different to bulk trains in that they generally run loaded both ways . A coal or wheat train has an empty leg and a fully loaded leg and the difference is considerable . Say for example you leave Pt Kembla with four 82s and 45 hoppers and bulk wants you back at Lithgow as soon as possible . 12000 Hp burns a lot of fuel and the trains don't handle as well with excessive horsepower . If they had the ability to be limited to 1 6 7 or 8 notch you can burn less fuel and set the power to suit the needs of the various locations on the fly .
For us the best example would be those long Perth Intermodal services up to 1800m and 5200 tonnes . It may take 4 NRs and 16000 Hp to get them going and up to speed but quite often just two will keep them going at the required speeds to run their tables . These longer trains if loaded evenly are great for momentum running and often don't need anything like rueling grade power to keep them stretched and at the required speed .
Fuel costs are a significant part of any rail operators bottom line - to the point wher more modern and fuel efficient units can be justified in budgets . The theory is that AC diesels pull more than DC units of about the same power and weight so if you use less of them running flat out more often you save fuel and money . There will still be times when ruleing grade power with AC units can be too much because its not like they can pull double what a DC unit can , I think the aim with these C44ACs is 200 t/unit more than NRs can so 3000T through loads for two units between Melbourne and Brisbane . The $64 question is can they run the tables with 8600 Hp .
First word so far is that two 93s want to pull at around 28 km/h with 1500T loads on 1:40 grades .
I think the first run of the 93s is on Saturday nigh out of Melbourne then back and forth to Sydney for a week .
Going to be interesting for people who've virtually lived in NRs since 1997 to see if the update C44ACs live up to expectations , time will tell .