On 4 July 1905, the line opened connecting with the Darvel Branch, resulting in the ceasing to become a branch and became a through line to Strathaven which was a jointly run line between the Glasgow and South Western Railway and the Caledonian Railway. However despite being a through line, no trains ever ran between Kilmarnock and Strathaven; instead, the two companies took it in turns to run the line between Darvel and Strathaven every six months.
The line was never successful and closed in 1939. Little evidence of the line still exists today, aside from the occasional embankment, bridge and cutting along the route.
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Lewin, Henry Grote (1925). Early British Railways. A short history of their origin & development 1801-1844. London: The Locomotive Publishing Co Ltd. OCLC11064369.
Stansfield, G. (1999). Ayrshire & Renfrewshire's Lost Railways. Ochiltree: Stenlake Publishing. ISBN1-8403-3077-5.