the line was one of the most heavily engineered in south wales. it cut across the grain of the landscape and involved numerous curves, steep gradients, tunnels and viaducts. it branched off the newport, abergavenny and hereford railway near abergavenny, crossing the river usk on a flimsy viaduct adjacent to the road crossing, and began a steep climb at gradients as severe as 1 in 34. after the line meandered around the blorenge mountain through the clydach gorge, climbing upwards on a breathtaking ascent at gradients of 1 in 38, with the upper section hewn out of a hillside shelf. beyond , the line rarely descended below above sea level, crossing despoiled treeless moorland and the heads of the mining valleys to dowlais. signalled the start of a steep descent of 1 in 40/50 to morlais junction and then over the brecon and merthyr (b&m) to merthyr.