The question is whether you could run diesel freights through an approx 22 km tunnel (Thirroul-Waterfall) or would you need to open the M-D or keep the existing line for them? All very costly solutions which I'm sure are being examined by the government's intercity study team. With the existing line in place and the tunnels mainly built for the commuter traffic, I wouldn't have thought the tunnels need to be built for diesel traction, maybe going down hill is ok (no idea), but why when the existing line can effectively become the Sth Coast Freight Line.
It's a problem so complex that no public discussion forum will come up with a simple answer, but my other point is that, in the meantime, we should be looking at the 80% of the line that is pretty straight and free of severe grades (for electric trains) - the sections between Bomaderry and Thirroul and between Waterfall and Central. I've done a bit of analysis in the past applying either Sydney metro or Perth system timings (both pretty identical per distance per number of stops) to these sections and this tells me that Sydney-Waterfall (38 km) could be taking about 40 minutes on a suburban (no interurban trains stop there) and Thirroul-Bombo (47 km) about 40 minutes on an interurban. The present journey times for these sections are about 50 minutes in both cases and you can understand why when you ride them. They amble along like Browns cows, hang around stations, accelerate and decelerate slowly etc.
I agree that full duplication south of Coniston would help, but the point is that these slow journey times also happen on the duplicated line to the north. The trains still amble. That's the big issue - the lack of performance of the existing trains. How to solve that?
You could not rule out the aspect that ARTC or other would want use of line if they wanted a complete shutdown on the other one for extended maintenance/track work.
Thing is with diesel operations and loaded trains on the down hill grades, the primary training these days appears to be that rather than the use of air brakes for control of the train, Dynamic brakes are used especially on loaded trains, the efficiency of that sort of operation and power of the modern generation of diesels means that a train can be pretty much brought to a stand with the use of the dyno's exclusively. I have nil idea regarding the working of steel trains and the like from Port Kembla heading north, but unlike my time when we had 48's as the primary up loco's heading to Enfield with max loads 1350tonnes and usually at least 3 such trains in the evening, most likely only one train would be the norm these days.
The aspect regarding the current line is that of the curves from memory, all those curves were in the main 60-80Km/h closing to 55 around the Lilydale area, I will look up my old WTT's to see what they were.
Another aspect that I have noticed these days is the amount of clearance that is generally found between the track and any cuttings or embankments, when the 81's were first brought to WCK when after I had transferred there, it was incredible the ground vibration that they created, something pointed out to me by the signalman at Pangella where we sat waiting for a down train to arrive, I was in the box and felt the floor vibrating, the signalman said there was an 81 on that down train, as it got closer and you could hear it the vibration was quite high. During that time and their use over the range there were quite a number of locations where the embankment sides would have rocks of different sizes dislodge and roll down to the side of the cuttings/embankments.
Using Google Earth, I can see a fair amount of widening has been carried out, no doubt much as a result of the O/head going up and needs for road vehicle access. Similar work is on show on the Short North as well.
If one considers the way trains work to/from the SC, the primary loaded trains are on the down while empties on the up, with a few loaded ones thrown in on the up. With that into consideration and even on the M-D line the same would apply, the up loaded would have dynamic brakes for all the trains going down the line, and at regulated speeds. Empties would go up much faster, and depending on how many donks were up front, it would be possible to drop at least one off line, moreso the last one in the consist as was done on the Ulan line in the early 81class days before the air intakes were improved/relocated.
As I look at the aerial view high enough to show the M-D line, also the prospect of the Metro from Carlton - Miranda - ? and a new line from Waterfall to connect where I believe would be the ideal spot to be just north of Austimer or Thiroul, given the overall costs especially between the Metro & HR project I would suspect that the HR would work out the least cost.
Either project will be expensive and has the advantages of removing many cars off the roads one thing for sure though, I will not see any of those projects started let alone finished.
One thing that would also a priority would be the realignment as far as a new station for Helensburgh, that would mean a much straighter line from Waterfall which would save a lot of time for both passenger and freight services. Helensburgh is pretty much a growing area as against Waterfall and other stations from there to Central.