Are you referring to the line that diverges at Mauchline then goes a short cut across to Ayr?
I had a quick eyeball on Google Maps last week, and it appears that the lines to the yard near Newton Jct (Ayrshire) on the left hand side facing Ayr are not connected. However, it seems to connect to the main line in the Paisley/Glasgow direction just before Falkland Jct.
I have thought about if Mauchline station ever reopens, the existing Carlisle - Dumfries short workings could run to Ayr. However, I have not used the Automobile Association website to see how Ayr - Carlisle takes by road for comparison. Maybe a new thread if a lot of responses?