Lets consider some things in regard to the BX & other comparable lines. ATM I cannot compare the Bomaderry services with anything as the CR web shows buses replace trains from Bomaderry to Wollongong & only a bus at 0330 to Kiama is shown.
Bomaderry 153Km's from Sydney Terminal. While having a change of trains at Kiama, is serviced by a fairly regular service although with the CR site not available I am only basing that on previous looks.
Goulburn 224K's from Sydney, does not appear to have any great service but works in conjuction with MV - Campbeltown service in regards to CR t/t's but also has availbe a couple of Canberra & the XPT services to them, those between MV & GLBN are not likely to have access to those trains though. But I would think that only Bundanoon & possibly Marulan would generate any sort of patronage anyway.
Moss Vale. 145 K's to Sydney, Despite its service being a change at Campbelltown, has an almost hourly service which is on par with many other locations at the same distance, & even though served by EMU's Newcastle services are often a change at Gosford, & Morriset services often have no connections for the Sydney end.
Lithgow 155K's from Sydney. after a couple of early services only has a 2 hourly off peak run, although through trains.
Bathurst. 239 K's from Sydney. The only rail service is by the XPT & maybe the cars on the IP/Ghan, or bus/train service. Outside of Bx the only potential for passengers is likely to minmal at Tarana & who knows with Wang? CR timetables do not show any connections in their online timetables to buses. The only advantage they will get is a through service.
Train useage. While this service means an updated train, the worst of it is that it has a lot of sitting time unless something can be squeezed in on arrival at Sydney Terminal & its afternoon return, where it sits overnight at BX. But, how many sets sit o/night at MV & where ever the Bomaderry sets are laying over?
From what I see of this, & in the end it sits in with what thadoctor says is the Bx passengers get a through service whereas the others do not. Bx is the longest trip by some 15K's than Goulburnm & at least 80k's further than the others, & 94 from MV.
The thing is though, I see the Bx service as more testing the waters, & there is time before the next election if it fails to can the service, without any real political fallout.
In the end, though even if the service continues it will still only provide a single service each way when all the others have multipe & frequent services, despite the need to change trains. What is better, a frequent changing service as against a less frequent through service, which may end up happening, as it may well be determined by patronage anyway on all lines.