These two cities are separated by the rocky uplands of Dartmoor forcing the early railway surveyors to propose that the line skirt the difficult terrain of the comparatively sparsely populated moorland.
Isambard Kingdom Brunel, in surveying the South Devon Railway, opted to push a line along a coastal strip between the Exe and Teign valleys, and then to climb the southern outliers of Dartmoor making for the head of the Plym estuary.
The seventh steepest main line bank on the British mainland, with a constant 4+1⁄4 miles (6.8 km) initially between 1 in 45 and 1 in 70 before easing to 1 in 90 and then increasing to 1 in 65.
This practice continued after the change to standard gauge, with Dean 4-2-2's handing over to 4-4-0 Duke class engines for the climbs, and later Bulldog 4-4-0s taking over from the Atbaras with their 6-foot-8-inch (2,030 mm) driving wheels.
With the advent of the HST, the power-to-weight ratio of 4,500 hp (3.4 MW) to 374 long tons (380 t) meant that there was no difficulty in keeping to time.
Much of the reason for this change in policy is the relative non-availability of standby diesel locomotives on the modern railway, where minimal freight traffic operates in the south west of England.
Many tests and comparisons carried out by the GWR and BR(WR) have involved traversing the South Devon Banks.
Performance up Dainton and Rattery was deemed satisfactory, but four subsequent test runs were carried out ascending Hemerdon bank.