Not only possible but common. The two lines share one of three rails, with two others for the narrow and standard gauge. There was dual gauge track across the Merivale Bridge since it was built, and south from Park Road as far as Bromelton near Beaudesert since QR converted the standard gauge for the narrow gauge third road. There are numerous other Australian examples in the
Wikipedia entry.
@Melburnian1 is correct. The ridiculous timings into Brisbane - one hour more ridiculous in summer with daylight saving - are because the Merivale Bridge is a choke point and fully utilised. It has constrained the building of proposed new commuter lines. Haven’t seen anything about the impact of cross river rail on timings, as the XPT isn’t on Queensland’s radar like commuters are. CRR entrenches the dual gauge line as a commuter line by
building platforms on it, but it will relieve pressure on Merivale Bridge. I’m hoping it would enable a middle of day arrival and departure at Roma Street.
cheers skip