The X class locomotives were introduced to service in 1966. They were built by Clyde Engineeering with EMD components and are an EMD export model G-16.
There were three types placed into service:
The units saw service on all main lines on both passenger and freight. Traralgon running was extremely rare until the end of electrified service in the early 1990s.
Until through train running on the standard gauge, several X locos were assigned for running between Albury and Melbourne.
X 31 was notable for running on the standard gauge for many years. The auto staff exchange box under the cab floor was to incorrect dimensions and so was not run on broad gauge. Only in the late 1980s with run through locomotive power did the loco see service on broad gauge after equipment modifications.