Hi all,
Couple of things. First of all I'm a Perth Train Driver with interest in all things Strathmore so hopefully I can put my oar in here!
The double line south from Stanley Jn to Perth is not bi-directional. It is a conventional up and down arrangement with an arrangement at Stanley on the up line to allow trains (machines) to shunt clear of the single line and return to the single line using fixed signals. However a short section of bi-di does exist between Perth New Yard (site of) and Perth along the down line.
On the subject of Strathmore traffic. All main line west coast (from Euston), all ex CR / LMS and BR Glasgow traffic (passenger and goods) from the central belt (and intermediate points) to Aberdeen ran via the Strathmore line. The Strathmore line was the trunk main line. It was the Perth to Dundee line that was the branch, becoming the main line after 4th September 1967 after closure of the Strathmore route.