Then add in the problem of mixing suburban/interurban trains on the same tracks as they did in the 1990's.How many freights per day on the Belair corridor?
Incidentally there was another runaway truck crash at Mt Osmond just this morning - it really is a fairly frequent occurrence.
Mt. Barker in this case not Bridgewater - so it might be more viable.
Probably Balhannah as well as Bridgewater, Aldgate and Stirling should all be stops but in theory Belair the last stop until (say) Goodwood. Although they're going to get stuck behind and between freight trains aren't they.
Rail has had multiple obstacles in this state not the least of which the gauge mix situation.
Any idea of nominal timing of these trains?
Mt Barker service would be no more than a handful of trains per day, targeting peak and midday.
Gauge issue is only minor, not every train everywhere can and does run through the city. For example nearly all Southern Highlands NSW services terminate Campbelltown. Sydney also has the Metro which for 6 years won't run into the city and before that we had Carlingford as well as other services where changes are required to access the city station. Brisbane has or at least had a few of its own examples.
Cross platform change at Belair connecting to a express service would be relatively painless or Goodwood which has cross platform to/from the city connecting with Belair line trains.
The more we look at this the easier it is to see a low cost trial is not that hard to bring to the table.
I've heard it suggested that Blackwood would be a better platform transfer point than Belair due to bus connections to FMC/Marion etc, which I think has some merit.
Acknowledging my pro-rail bias, aside from the cost of accessing ARTC track I don't see too many real issues having a MB -> metro connection using the 3000s. Examples of passenger numbers from the late 80s are not an apples to apples comparison (population growth + trains only ran to Bridgewater); and while the duration of the journey will exceed that of a bus the whole point is that an incident on the SE freeway will blow out commute times to greater than that of a train. It's worth at least a trial.
Disruptions from SCT (and other) freight trains failing in the Hills are more frequent than major hold ups on the freeway.
You still have to convert Mount Barker to Mount Barker Jct to standard gauge and reinstate a connection at MBJ
A cross platform interchange at Belair will see the ARTC crossing looped blocked.
A cross platform interchange at Blackwood will require construction of a passing siding
Mt Barker station would have to be converted to dual gauge with disruption to Steamrangers very successful heritage operations.
You and RTT_Rules say you want trial, but all the above have to be dealt with to even get to that stage. Tell us how much it is all going to cost? I doubt you would get much change out of $40m just for your trial, so it's worth bearing in mind that the state government recently scrapped the $40m Port Adelaide extension project because it cost too much.