So why start with 7 carriages? If they have to add a few extra carriages anyway when MM1 opens then why not just run 6 cars in the interim and avoid all the hassle 7 carruages has caused?
the clue is in the name of the projectHigh Capacity
7 carriages carry more pax than 6.
Those 7 carriages already do fit on the Dandenong line
the City Loop platforms are a bit short.
Hence the signals have been moved a couple of metres into the tunnel so the drivers nose can poke in.
A whole thread about the plan to run the Dandenong loop anti-clockwise all day, to make that alteration work for HCMT.https://www.railpage.com.au/f-t11400462.htm
Hence the new trains are now waiting on a re-vamp of the Metro Time Table - Jan/Feb is the rumour.
When the Metro tunnel is opened for Dandenong/Sunbury, then the planned 10
carriage trains can run.
All the new stations on the line have provision for platform extension.