Why would that mean extra empty cars to Dubbo? Isn't there a daily service to/from Sydney? One unit shunts out, another unit docks up ex maintenance.
You may not understand how many things can go wrong with heavily utilised railcars, as AussieJasmine sagaciously points out.
At present, because the Marrickville depot is close to Central railway station, NSW TrainLink does a pretty good job of maintaining the ageing, yet still fairly reliable, fleet of XPTs and XPlorer railcars. Both these trains run an incredible number of kilometres per annum, yet recently when I was on the XPT, it still had no trouble quickly reaching 160kmh when permitted by track maximum speed limits. Similarly, the XPlorers easily reach 130kmh as I also recently found.
The decision to put the maintenance facility in Dubbo was made because at the time the NSW Coalition Government was at huge risk of losing that seat. Many may like Coalition Governments but this decision was lunacy.
I don't know how they're going to get railcars 'empty cars' (i.e. without passengers) from Sydney to Dubbo if the set has failed: perhaps haul them with a locomotive up front, but that may need a special transition coupling. It'll be seven hours or more with a slower 'empty cars' train up the Blue Mountains and via the windy track to Orange, and then straighter and faster to Dubbo.
It will inevitably mean more long distance trains become unpopular road coaches, which can sometimes be much slower than trains (particularly if they have to go in and out of each railway station) and are disliked for their general lack of comfort, such as constricted seating, inability to move about as one can on a train, a coughpy service station 'refreshment stop' compared with trains' pleasant on-board buffet, and well, road coach toilets...