I'm told it went there in order to allow X'Trapolis trains to use the line, the trigger was new train wash being built at Craigieburn Yard.
Passing through the platform at Kensington: XTrapolis trains have already been cleared as far as the racecourse, so they didn't need to check this platform.
Crawling along at walking pace at Ascot Vale: staff following alongside checking the platform clearances.
At each station the train would stop short of the platform, then let the staff out. The train would then crawl along to check the clearances until they reached the departure end, then they would pick up the staff again.
Inspecting the platform at Strathmore