Superelevation

Superelevation or cant refer to the practice of laying the outside rail on a curve higher than the inside rail. This helps steer trains around the curve without the flanges touching the rails. The result is reduced wear and improved passenger comfort. On the pre-revival Festiniog Railway cant was an essential track feature for the efficient running of gravity slate trains.

There is a formula for calculating the ideal amount of superelevation according to the gauge, speed of trains and the radius of a curve - it is called the Molesworth formula:

$E_{a}={\frac {Wv^{2}}{gr}}$.

Where $E_{a}$ is the amount of cant required, $W$ is the gauge, $v$ the speed of the train, $g$ is the acceleration due to gravity and $r$ the radius of the curve. For the required cant measured in inches, the gauge and curve radius in feet and the speed in miles per hour this formula becomes

$E_{a}={\frac {Wv^{2}}{1.25r}}$.

The FR's tightest curve is Tyler's Curve, it was formerly 1.75 chains but has been eased to 2.4 chains. The ideal superelevation at different speeds according to the Molesworth formula are:

• At 1.75 chains radius, 1.39 inches at 10 mph, 3.12 inches at 15 mph and 5.54 inches at 20 mph.
• At 2.4 chains radius, 1.01 inches at 10 mph, 2.28 inches at 15 mph and 4.05 inches at 20 mph.

In practice less superelevation is applied to allow for slower and even stationary trains. At sharp curves on the FR superelevation was partly achieved by using special deep chairs on the outer rail ("high heeled" according to Will Jones). The high superelevation on FR curves has frequently been criticised by the Railway Inspectorate. See for example the report of Major E Druitt on a derailment at Coed y Bleiddiau in 1901. In addition excessive cant was criticised in the report of the inspector on the reopening to Boston Lodge in 1955 and the reopening to Penrhyn in 1957. The inspector was limiting the speed on the unrelaid portions of the two sections to 8 mph and 10 mph respectively and the two sharpest curves on them were Boston Lodge and Tŷ Fry. In his 2016 article referring to FR track in the nineteen sixties Fred Howes comments on "a ridiculous excess of cant".

Since the revival the superelevation on curves on the FR has been reduced. Careful realignment of curves has also been used to increase their radius as at Tyler's.

Superelevation is measured with a spirit level with an adjustable scale at one end - called a "cant gauge". These cant gauges are more generally used to measure cross levels - the difference in height between pairs of rails. They were not available to platelayers of the pre-revival FR and very rapid changes of cant (cant gradients) were used to achieve the large amounts of cant on the old FR on the many reverse curves. The modern maximum design cant gradient for new track installation is 1 in 720. Measurements in the 1960s showed it be as much as 1 in 256.