PN got the contract when the G&W CLPs proved too unreliable.
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The CLP had successfully handled all 3 ANR long distance trains from their introduction in 1992, the Overland in 1995, until the day GSR took over. With head-end power it was suited to those trains.
The final ANR train from Melbourne to Adelaide was hauled by a CLP and the first GSR train to Melbourne that evening by an NR, and incidentally minus the club car and plus a power van.
At the sale of ANR in 1997 there was only one national operator, National Rail. The only other operator in SA was startup ASR which had a fleet of ageing 800 HP and 1600-1800 HP locomotives the youngest of which were the 700 class. Even these were 26 years old.
ASR had a fleet of 25 more modern higher horsepower locomotives: the ALF and CLP/F classes, rebuilt in 1994 and 1992 respectively so effectively only 3 to 7 years old.
The other more modern higher horsepower locomotives, the BL, DL and AN classes has passed from AN to NR, and the EL class was excluded from the sale.
All the other operators in Australia were state-based government systems.
If ASR was reluctant to make an offer the only logical operator for GSR was NR.