Yes, the Buxton line is steeply graded. The summit is at Dove Holes. It's a stiff climb to Dove Holes from either side. I can't remember the figures but steeper than 1:100, on a curvy route and one that often has wet rails thanks to hill country weather. I think Pacers simply lack the traction to be reliable, so they're banned. I remember Pacers being tried on the Manchester-Glossop/Hadfield line when there was a shortage of EMUs once. Glossop is a bit like Buxton - a steep climb out of the terminus on curvy track, often wet. The Pacers couldn't cope, so they're banned from Glossop too, but as it's electrified that's not normally a limitation.