The initial proposal had been put forward in 1846 but the scheme was not passed until it received backing from the London and North Western Railway.

[2] A further Act of Parliament was passed in 1862, giving the jointly owned Llanidloes and Newtown Railway (L&NR) (which was managed by the Cambrian Railways since 1860) the rights to extend southwards from Llanidloes with 1.5 miles (2.4 km) of double track to Penpontbren Junction where the M&MR Llangurig branch would diverge through the Cambrian Mountains to Strata Florida and the MWR line would continue to Builth Road (serving Builth Wells).

[1] The branch line was officially 'opened' on 28 October 1863 comprising approximately 4 miles from Penpontbren Junction to Llangurig station where there is no visible evidence that platforms were provided.

This legally entitled the L&NR to invoice the M&MR for its share of the cost of building the joint junction station at Llanidloes, which it promptly did.

[1][2] By 1875 the M&MR was in receivership because it had been unable to raise the capital required to complete the mountain section of the railway west of Llangurig or arrange running powers from the NW England to enable traffic to reach Milford Haven.