To deal with congested trains on the South Coast line, they made them longer - but then forgot about the platforms.
In February, Transport Minister Andrew Constance announced the purchase of an extra 42 carriages for the New Intercity Fleet (NIF).
Most of those would be sent to the South Coast line to turn the eight-car express trains into 10-car sets.
Those trains will still hold fewer passengers than the eight-car trains now on the South Coast line - but a planned boost in service frequency will cancel that out.
However, a problem has surfaced with the platforms the express trains will be stopping at - at 204 metres, the trains are too long for most of the platforms.
That means Transport for NSW has to carry out work to extend a number of platforms on the South Coast line.
The first in line is Wollongong, where the southern end of both platforms will be extended by nine metres, according to the Review of Environmental Factors.
"It is proposed to operate 10-car NIF trains on the South Coast line, which would be longer than the eight-car Oscar trains currently operating on the South Coast line," the review stated.
"An extension of the platforms at Wollongong station is required to meet operational requirements and to enable formation and division of 10-car NIF trains.
"It is an operational requirement that all doors of a train, including the driver's door, must be able to open to a platform to allow all customers and the driver to egress."
Work is also planned for the stabling yard south of the platform.
"Transport for NSW proposes to upgrade the stabling yard to accommodate up to eight 10-car NIF trains concurrently," the Review of Environmental Factors stated.
The work is expected to start mid-2020, subject to approval and take around 18 months to complete.
Most construction work would be done during periods of weekend trackwork.