Civic is the former station on the Newcastle rail line. The current Canberra railway station is Kingston. The present official route for the Canberra Sydney high speed rail shows an underground station below Ainslie Avenue adjacent to the shopping centre.
Goulburn only gets one direct Southern highlands peak hour nsw trainlink intercity service to Sydney each way a day. Goulburn also gets a few southern highlands services to Campbelltown a day, the 3-4 Canberra services a day, the 2 XPT services a day and the once a week Griffith service. So what services are you going to cancel?
My only issue with your idea is that it would leave Queanbeyan without a rail service to Sydney and considering that is the NSW city in the area. I don't think your idea is workable from a NSW trains point of view. Fixing the track and even the alignment to run alongside the kings highway instead of following the border alignment should help reduce travel time significantly without the need to build an expensive new rail line to the airport.
If the operators of Canberra airport want to build such an alignment then they should put the money up for it and look at operating trains to Sydney themselves. The light rail is also proposed to go to Kingston as well as the airport so that takes care of that argument in any case.
The post office in the shopping centre is called Civic Square to differentiate it from the Newcastle suburb.
From the station, the line would tunnel under Mt Ainslie, and follow the Majura Parkway up to the border, then follow Gundaroo Road to a junction located on a ne line a bit south of Gunning.
This puts the line adjacent to Canberra Airport at the point it enters the tunnel and it would be easy to extend the line to Canberra Airport, and building it to the airport first would save the cost of the tunnel and the underground station if a cheaper alternative were required.
Canberra Airport is actually about the same distance from the city centre as Kingston. Kingston is closer to Parliament House but I don't see Parliament house as a major destination for rail travellers, at least not until they have left their baggage at their accommodation.
Building that high speed line (just the branch to Canberra) would reduce the journey time to Goulburn to less than half an hour but would bypass Queanbeyan entirely.
There is a direct road from Queanbeyan to Canberra Airport which would allow passengers from Queanbeyan to drive there in about ten minutes more than driving to Queanbeyan Station which is a long way from Queanbeyan's CBD (maybe further than Kingston is from Civic.)
Ok lets call it Kingston to be clear, but yes Civic is a location near the current station that at times comes up.
There are four or so extended bus and trains a day beyond Moss Vale to Gouburn, obviously they would be cut/curtailed if an enhanced service to/from Canberra.
Going direct to say Kingston from Bungendore, you save around 6-7km and more favorable terrain and you basically go around the back of the airport on southern side. If you go to a station say under the main terminus of the airport you save around 10km although a tunnel is involved unless you come around from the north.
From the Airport you can enter more closely to Canberra if there is really a need fairly easily and this is the question. Yes you will get closer to the centre of Canberra but closer to what? Still too far to walk to 99% of the greater city of Canberra which is over 30km across and mostly a low density city with island communities. I doubt there is an ideal location where this is resolved at all. Getting next to a Light Rail station is probably as good as its going to get.
Now I know Sims has this love affair with this concept that he reads my comment on building the line to the airport and see's "lets do this for Canberra Airport's sake", but that's not what I proposed it for. If you are going to spend $1B or more upgrading the line to Sydney so much that you are going to run a reasonable train every 1hr or 2hr at the most, a shed in a park in the southern end of Canberra is not the destination. Right now most users are OAP, welfare, students, a few locals along the line and the odd train buff and tourist for which the current arrangement is "OK".
If you are going to be pulling people out of plans and cars in large numbers, from a few hundred an hour in peak every 2hr in off-peak, you are getting a different level of clientele. Additionally by the time its done the Light Rail will be at the airport. The Airport has far more than Kingston will ever offer. More bus services, LR, car parks, food, more taxi, hire car, hotel transfers, connection to plane etc etc.
As for Queenabeyane, yes cutting the train down there will have some effect but not much. Again how many use the service now? We are building MSR to attract a few thousand users a day, targeting more business, political and tourists, the NSW govt wont' fund this upgrade on its own at all, it will be funded by Fed/ACT govt especially as the bulk of the route to Sydney is on Fed funded tracks. What the few NSW voters down that way loose in local proximity they gain in speed on the train so overall no loss and probably only more to gain. The current service crawls from Kingtston to Queen'ie and then runs straight into significant terrain, it won't be easy to fix and will ultimately cost travel time. Run the LR down to Queen'ie from the airport for a fraction of the cost and that put them back in their box and keep them quiet.