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The Festiniog Railway has many very sharp curves as it was built to follow the contours of the hills, which tends to reduce construction costs.
On the other hand, sharp curves limit speeds, and increase wear and tear on both track and wheels. To somewhat counter that and the centrifugal force, the railbed is usually slightly canted inwards or the outer rail is superelevated over the inner. The cant must never be so steep that a train cannot stop safely in the curve.
There are also sharp curves at turnouts (also known as points or switches), but the radius of these is not well documented.
As on most railways and some tramways, the entrance and exit to a curve is eased by its being parabolic, which means that the radius is wider at the ends of the curve and tightens towards the middle. The radius given in such cases should be the tightest. These are also called transition curves. 
There are also some global speed limits that apply to both curves and straights; these are marked as 999 chains radius so that they sort correctly.
- (WL: gromlin)
- Note: 1 chain = 66 feet = 20.1168 metres.
|101||1.75 (as built)
|6||Tyler's Curve||Sharpest curve on main line,
around the hanging valley of Llyn Mair
|102||2||Groby Junction||Curve from junction||FR||Z|
|201||7||Day Log/1896-08-28||Mounted rail and derailed.||FR||U|
|202||7||Day Log/1896-10-07||Further to incident of 1896-08-28||FR||T|
|203||4||Day Log/1901-10-04||Derailment. Too much super-elevation (or cant) 3in.||FR||S|
|301||2.50||Afon Glaslyn Bridge||WHR||E|
|303||Coed y Bleiddiau||Deviation to ease (increase) radius||FR||DV|
|304||2.50||Goat Tunnel||curves at either end of tunnel ; speed restriction||WHR||TN|
|John S Hughes/Interview||Sharpest curves||FR||CV|
|411||6||12||General minimum radius according to
faulty Antipodian newspaper in 1872
|451||4 (as built)
|890||0.45||0||02||Smallest curve by Fairlie locomotive at slow speed||CV |
|898||2||2.50||8||FR||General minimum radius curve||FR||FF |
|901||WHR||General minimum radius curve||WHR||GG|
|905||999||12||FR||1875 General speed limit||FR||XX |
Sharp Curves (raw data)
- Tyler's Curve - is the sharpest curve on the line, where up trains change direction from North-East to North-West around the hanging valley of Llyn Mair
- Afon Glaslyn Bridge - Portmadoc Beddgelert & South Snowdon Railway on yet another curve of about 50m or 53m radius. This was originally laid with wooden sleepers in the belief ...
- Coed y Bleiddiau - The alignment of the Railway has been modified here to increase the radius of the curve by moving the line outward on an impressive embankment.
- Day Log/1896-10-07 - However, the incident happened on a right hand curve of 7 chain radius. The straight sections immediately before and after this had been ...
- Goat Tunnel - introduced and the track lowered on steel sleepers, but there are still curves of some 50m radius at either end of the tunnel, so there is a permanent speed restriction.
- Day Log/1896-08-28 - west of the former station a right-handed curve of seven-chains radius, and it was while travelling round this curve that the engine mounted the rails towards ...
- Groby Junction - from the junction at about 1 in 25, turning on a left-hand curve of about two chains (40m) radius, crossing the Glan y Pwll - Tan y Grisiau road (formerly ...)
- Carriage 15 - also in order to take the strain of a train passing through reverse curves of short radius, and to haul large rakes of empty wagons. The bogie centres and ...
- Beddgelert Station - Bryn y Felin easing the gradient to 1 in 40 building three curves of barely more than 50m radius between Bryn y Felin and Beddgelert some quite odd alterations ...
- Croesor Junction - extended towards Beddgelert — it curved away north-westwards on a low embankment at a tightish curve of 4 chains (80m) radius, slackening slightly to 6 chains,
- Carriage 16 - also in order to take the strain of a train passing through reverse curves of short radius, and to haul large rakes of empty wagons. The bogie centres and ...
- Day Log/1901-10-04 - train was running at the time of the accident on a left-handed curve of 4 chains radius, and with 3 ins. of super-elevation (or cant) on the outer rail. The inside ...
- Dinas Branch 1976 - inside of the diverted FR line. It is said that the curves left in the slate waste are too tight a radius but it would not require any major effort to ease ...
- John S Hughes/Interview - from the straight or reverse curve. The sharpest curve is 1 3/4 chains (115ft) radius on a gradient of 1 in 59 - other curves of 3, 4, 5, and 6 chains.
- "REVIEW OF THE REPORTS ON VICTORIAN RAILWAYS.". The Age (5488). Victoria, Australia. 15 June 1872. p. 6. Retrieved 21 April 2017 – via National Library of Australia.
- "Narrow Gauge.". Illawarra Mercury (Wollongong, NSW : 1856 - 1950). Wollongong, NSW: National Library of Australia. 25 August 1865. p. 4. Retrieved 1 November 2015.
- National Library of Australia
- "Tasmanian Main Line Railway Company (Limited) Engineers Office". The Cornwall Chronicle. XXXIX, (4543). Tasmania, Australia. 3 September 1875. p. 4. Retrieved 7 April 2017 – via National Library of Australia.