Right, looky here.
For a SYD-BHS-SYD daytrip, you have two options. Option number one Dubbo XPT. Departs Sydney Central 0710, arrives Bathurst 1043; departs Bathurst 1707, arrives Sydney Central 2049. The timing is perfect for a daytrip, giving you six hours and a bit of time in Bathurst; you travel in reasonable comfort.
If you're too much of a cheapskate to cough up for that, here's option number two you can have a cup of concrete and take the CityRail service. This consists of an electric interurban to Lithgow, connecting to a coach onwards to Bathurst. There are several such services daily, so take your pick.
If you really, really want a CityRail day train, start a campaign for it and put your heart into it. You never know, you might see it made reality one day. Right now, you have two options convenience and comfort, or cheapness. Take your pick.
Cough up, harden up or stand up. Your choice.
Watson stop with the 3 letter stuff ok not everyone knows where you mean and its lazy.