When an engineer has to construct a railway up a hill having a still steeper slope, he must secure practicable gradients by laying out the line in ascending spirals, if necessary tunnelling into the hill, as on the St Gothard railway, or in a series of zigzags, or he must resort to a rack or a cable railway.
It was obvious Donald, and to a lesser extent Dean, would enjoy steeper, longer and more challenging runs.
"Suppose the stairs get steeper?" suggested Zeb, doubtfully.
Sometimes, however, a sharp incline occurring on an otherwise easy line is not reckoned as the ruling gradient, trains heavier than could be drawn up it by a single engine being helped by an assistant or " bank " engine; sometimes also " momentum " or " velocity " grades, steeper than the ruling gradient, are permitted for short distances in cases where a train can approach at full speed and thus surmount them by the aid of its momentum.
Abt also developed the plan of combining rack and adhesional working, the engine working by adhesion alone on the gentler slopes but by both adhesion and the rack on the steeper ones.