The Parliamentary Bill for this was passed on 6 August 1866; GWR opposition was in vain in view of its obstructive attitude; the Act authorised an additional £115,000 capital. Seeing that its position was lost, the GWR now agreed to negotiate, and an agreement was reached on 18 July 1866. The line from Whitland to Carmarthen was double track, and instead of providing mixed gauge track, the GWR agreed to convert their up line to standard gauge between Whitland and Carmarthen.