The new 10-car trains - designed to alleviate crowding on express services - will be too long to fit at some stations.
Also, the platforms on two of the express stops - Unanderra and Helensburgh - are so short that it cannot even fit the eight-car trains that stop there now.
In February, Transport and Roads Minister Andrew Constance announced the purchase of an extra 42 carriages for the New Intercity Fleet - most of which would sent to the South Coast line.
The extra carriages would in part be used to increase peak-hour express services between Wollongong and Sydney to 10 carriages to alleviate the commuter crush.
According to Transport for NSW a 10-car New Intercity Fleet train is 204 metres long.
That's longer than the platforms on almost every express stop in the Illawarra.
This includes Unanderra and Helensburgh - both of which are already so short Transport for NSW designates them SP6, which means passengers can only get on and off via the last six carriages.
Transport for NSW documents state Helensburgh's longest platform is 164 metres - 40 metres shorter than the new express services.
It's worse for Unanderra at 139 metres - that means one-quarter of the 10-car trains won't fit on the platform.
Coniston - also an express stop - is 179 metres long. That's 25 metres shorter than a 10-car train.
If passengers at these stations will not be able to board or alight from some carriages, it raises questions about the 10-car trains' ability to reduce the commuter crush.
Transport for NSW has said it planned to extend some platforms in the Illawarra, including Wollongong and Thirroul.
However, Helensburgh and Unanderra are not among those stations - the new technology in the fleet means the doors of those carriages off the platform will not open.
As for work on the platform extensions at other stations, that is due to start next year.
"Platform extension work will be staged and is expected to commence in 2020," a Transport for NSW spokeswoman said.
"The work at Waterfall station is on public exhibition and Wollongong station will follow in the coming weeks."