The colonies here had different rail gauges and you used to have to change trains at the border to get on a local sized train. Since Federation (1/1/1901) they found that wasn’t a great idea for fostering trade among the States (which is what the Colonies became). Took years and years but now the Interstate tracks are all the same guage. i think the local cities have kept their odd sizes. NSW I think is standard 4'8.5". But the Govt is rushing a "Metro" into Sydney which I think is standard gauge but driverless and promising service every 4 or so minutes. Started with serving areas that never had ‘heavy rail’ but now about to replace a (relatively short for Sydney) route.
From wiki.
In the 19th century, each of the
colonies of Australia adopted their own
gauges. However, with
Federation in 1901 and the removal of trade barriers, the short sightedness of three gauges became apparent. It would be 94 years before all mainland state capitals were joined by one standard gauge.
The three main railway gauges in Australia are
narrow:
1,067 mm (3 ft 6 in),
standard:
1,435 mm (4 ft 8+1⁄2 in), and
broad:
1,600 mm(5 ft 3 in); a slow progression towards unification to standard gauge has taken place since the 1930s.
Rail gauge in Australia - Wikipedia