I don't want to sound like @sims with "it can't be done", but IMHO a lot of people are looking at opportunities to improve the Canberra service with excessive optimism.
The new CAFs won't have substantially better performance than the Endeavours on the route.
The route is so slow, shaving over an hour of the transit time means it is *still* highly uncompetitive with driving.
The CAFS are a min 3 car set, so a more frequent 2 car train service won't happen (or if it does, not for long)
The route itself is seriously problematic. The Main South from Menangle to Cambelltown has 70+ curves, most 400m radius (Picton's 300 deg curve is 300m), and that largely sets the top speed. Joppa to Canberra is laid with lightish (I forget what weight) rail and steel sleepers. This sets the 140kph limit on the rare straight bits. Sections of the route are built to "temporary" standard. It's pretty clear from a look at the map that the ridge on the Kowen Forrest was clearly intended to have a tunnel. The curves are all 240m radius, or 12 chains - IIRC the minimum allowable in the early 1900s. Similarly with the Tarago bends, and the Molongolo canyon. The line was built to a budget.
The issue with straightening the route here - beyond the cost - is the rail line by definition forms the NSW ACT border. Re-route the line an inch north and it creates a constitutional crisis. South of the line is the Defence Dept HQ, with it's various layered security perimeters. Move the line south an inch and you create a national security crisis. There is the other problem the line serves the ACT (sort of), who don't vote for or pay for the service (but AFAIK they do contribute to the Fed Highway.
Basically, it's all "too hard". But equally, the political jurisdictional quirks mean it's too hard to scrap also, and it doesn't cost *that* much (roughly $20mil subsidy a year).