Stephenson followed the route of the existing Rochdale canal but at the highest point he chose to drive a Summit Tunnel through 3km of the hill-top - you can't run trains like canal barges up through a stair-case pf locks. It was in its day, the longest tunnel ever driven, a massive engineering feat: dug by pick and shovel through a geological slice of coal, shale and sandstone which are all comparatively soft. To keep the tunnel on track a series of shafts were surveyed and sunk vertically from the ground above
The bricks were certainly put through a stress-test (short youtube) on 20th December 1984 when a train loaded with a million liters of petrol derailed in the tunnel and ignited. The train-drivers were able to decouple the first three tanker-cars and drive them out of the tunnel, and the fire-brigade were on the scene from Manchester and West Yorkshire very quickly but they had to deal with something like 500 tons of burning fuel in an awkwardly confined space. They tried to hose down the fire until they saw the heat of the flames spalling off the outer layer of bricks. As the temperature rose, the safety valves on successive trucks opened and more fuel was discharged.