Internally, the IMUs differ from the SMUs, being fitted with larger seats, luggage racks and mobility-access toilets.

To provide rolling stock for the new Gold Coast line to Helensvale, in 1993 a contract for four 100 class units was awarded to Walkers Limited, Maryborough.

The 120 series were the final batch of trains manufactured by Walkers Limited before being absorbed by Downer Group, Maryborough in partnership with ADtranz, which was likewise taken over by Bombardier.

In 2004, Queensland Rail ordered sixteen 160 series units to provide extra rolling stock in conjunction with the upgrade and duplication of the Gold Coast line.

In early 2007, during safety checks, it was discovered that air-conditioning units on the trains were 10 millimetres too wide for the loading gauge.