The Crystal Brook to Merriton section was really a sop to the then Commonwealth Government. The Commonwealth had used consultants who proposed a new SG line from Crystal Brook to Adelaide basically in parallel with the Red Hill Adelaide line. The BG line was already of a high standard parts being some of the best track on the SAR so the SAR proposed gauge conversion the entire route to Pirie. The consultants were obviously trying to drum up business with a new rail line.
The SAR wanted to use the considerable savings to be used to Gauge convert Gladstone/Snowtown/Wallaroo with the change used to convert the Wilmington to at least Boolaroo Centre. Stupidity and penny pinching prevailed and the SAR were left with hotch-potch system which proved impossible to keep viable.