Minimum railway curve radius
The minimum railway curve radius, the shortest allowable design radius for railway tracks under a particular set of conditions. It has an important bearing on constructions costs and operating costs and, in combination with superelevation (difference in elevation of the two rails) in the case of train tracks, determines the maximum safe speed of a curve. Minimum radius of curve is one parameter in the design of railway vehicles as well as trams.
The first proper railway was the Liverpool and Manchester Railway which opened in 1830. Like the trams that had preceded it over a hundred years, the L&M had gentle curves and gradients. Amongst other reasons for the gentle curves were the lack of strength of the track, which might have overturned if the curves were too sharp causing derailments. There was no signalling at this time, so drivers had to be able to see ahead to avoid collisions with previous trains. The gentler the curves, the longer the visibility. The earliest rails were made in short lengths of wrought iron, which does not bend like later steel rails that were introduced in the 1850s.
Factors affecting the minimum curve radius
Minimum curve radii for railroads are governed by the speed operated and by the mechanical ability of the rolling stock to adjust to the curvature. In North America, equipment for unlimited interchange between railroad companies are built to accommodate 350-foot (106.7 m) radius, but normally 410-foot (125.0 m) radius is used as a minimum, as some freight cars are handled by special agreement between railroads that cannot take the sharper curvature. For handling of long freight trains, a minimum 717-foot (218.5 m) radius is preferred.
As the need for more powerful (steam) locomotives grew, the need for more driving wheels on a longer, fixed wheelbase grew too. But long wheel bases are unfriendly to sharp curves. Various types of articulated locomotives (e.g. Mallet, Garratt and Shay) were devised to avoid having to operate multiple locomotives with multiple crews.
More recent diesel and electric locomotives do not have a wheelbase problem and can easily be operated in multiple with a single crew.
- The TGR K Class was
- 610 mm (2 ft) gauge
- 99 ft (30 m) radius curves
- Example Garratt
- 1,000 mm (3 ft 3 3⁄8 in) gauge
- 25 kg/m (50.40 lb/yd) rails
- Main line radius - 175 metres (574 ft)
- Siding radius - 84 metres (276 ft) 
- GER Class 209
- 1,435 mm (4 ft 8 1⁄2 in)
A long heavy freight train, especially those with light and heavy waggons mixed up, may have problems going round very sharp curves, as the drawgear forces may pull intermediate waggons off the rails causing derailments. Solutions might include:
- marshal light and empty waggons at rear of train
- intermediate locomotives, including remotely controlled ones.
- ease curves or deviate line.
- reduced speeds
- less cant (superelevation), but this is unfriendly to fast passenger trains.
- More but shorter trains.
- problem is less severe with bulk say coal trains, where all waggons weigh the same.
- better driver training
- driving controls that display drawgear forces.
- New c2013 Electronically Controlled Pneumatic brakes have the potential to reduce errant drawgear forces, besides displaying even more information to the driver.
A similar problems occur with harsh changes in gradients (vertical curves).
Speed and cant
As a heavy train goes round a bend at speed, the centrifugal force the train exerts on the rails is sufficient to move the actual track, which is only held in place by ballast. To counter this, a cant (superelevation) is used, that is, a height difference between the outside and inside rails on the curve. Ideally the train should be tilted such that resultant (combined) force acts straight "down" through the bottom of the train, so the wheels, track, train and passengers feel little or no sideways force ("down" and "sideways" are given with respect to the plane of the track and train). Some trains are capable of tilting to enhance this effect for passenger comfort. Superelevation is not used tramway tracks. The superelevation can't be ideal at the same time for both fast passenger trains and slow freight trains.
The relationship between speed and tilt can be calculated mathematically. We start with the formula for a balancing centripetal force; θ is the angle by which the train is tilted due to the cant, r is the curve radius in meters, v is the speed in meters per second, and g is the standard gravity, approximately equal to 9.80665 m/s²:
Rearranging for r gives:
Replacing tan θ with what has just been proposed gives:
This table shows examples of curve radii. The values used when building high-speed railways vary, and depends on how much wear and safety desired.
|Curve radius||≤ 33 m/s
= 120 km/h
|≤ 56 m/s
= 200 km/h
|≤ 69 m/s
= 250 km/h
|≤ 83 m/s
= 300 km/h
|≤ 97 m/s
= 350 km/h
|≤ 111 m/s
= 400 km/h
|Cant 160 mm,
cant deficiency 100 mm,
no tilting trains
|630 m||1800 m||2800 m||4000 m||5400 m||7000 m|
|Cant 160 mm,
cant deficiency 200 mm,
with tilting trains
|450 m||1300 m||2000 m||no tilting trains planned for these speeds|
A curve should not become a straight all at once, but should gradually increase in radius over time (a distance of around 40 m - 80 m for a line with a maximum speed of about 100 km/h). Even worse than curves with no transition are reverse curves with no intervening straight track. The super-elevation (aka cant) must also be transitioned. Higher speeds require longer transitions.
As a train negotiates a curve, the force it exerts on the track changes. Too tight a 'crest' curve could result in the train leaving the track as it drops away beneath it; too tight a 'trough' and the train will plough downwards into the rails and damage them. More precisely, the support force R exerted by the track on a train as a function of the curve radius r is given by
positive for troughs, negative for crests, where m is the mass of the train and v is the speed in m/s. For passenger comfort the ratio of the gravitational acceleration g to the centripetal acceleration v2/r needs to be kept as small as possible, else passengers will feel large 'changes' in their weight.
As trains cannot climb steep slopes, they have little occasion to go over significant vertical curves, however High Speed 1 (section 2) in the UK has a minimum vertical curve radius of 10000m. High Speed 2, with the higher speed of 400 km/h, stipulates much larger 56000m radii. In both these cases the experienced change in 'weight' is less than 7%.
- The Australian Standard Garratt had flangeless leading driving wheels which tended to cause derailments on sharp curves.
- Sharp curves on the Port Augusta to Hawker line of the South Australian Railways caused derailment problems when bigger and heavier SAR X class locomotives were introduced, requiring deviations to ease the curves.
- 5-chain (101 m; 330 ft) curves on the Oberon railway line, Batlow railway line, and Dorrigo railway line, New South Wales, limited steam locomotives to the 19 class.
List of minimum curve radii
|1,435 mm (4 ft 8 1⁄2 in)||7,000 m (22,966 ft)||China||Typical China's high-speed railway network (350 km/h)|
|1,435 mm (4 ft 8 1⁄2 in)||5,500 m (18,045 ft)||China||Typical China's high-speed railway network (250 km/h~300 km/h)|
|1,435 mm (4 ft 8 1⁄2 in)||4,000 m (13,123 ft)||China||Typical high-speed railways (300 km/h)|
|1,435 mm (4 ft 8 1⁄2 in)||3,500 m (11,483 ft)||China||Typical China's high-speed railway network (200~250 km/h)|
|1,435 mm (4 ft 8 1⁄2 in)||2,000 m (6,562 ft)||China||Typical high-speed railways (200 km/h)|
|1,067 mm (3 ft 6 in)||250 m (820 ft)||DRCongo Matadi-Kinshasa Railway||Deviated 1,067 mm (3 ft 6 in) line.|
|1,435 mm (4 ft 8 1⁄2 in)||240 m (787 ft)||Border Loop||5,000 long tons (5,100 t; 5,600 short tons) - 1,500 m (4,921 ft)|
|1,435 mm (4 ft 8 1⁄2 in)||200 m (656 ft)||Wollstonecraft Railway Station, Sydney|
|1,435 mm (4 ft 8 1⁄2 in)||200 m (656 ft)||Homebush triangle||5,000 long tons (5,100 t; 5,600 short tons) - 1,500 m (4,921 ft)|
|1,435 mm (4 ft 8 1⁄2 in)||190 m (623 ft)||Turkey||Turkey|
|1,435 mm (4 ft 8 1⁄2 in)||160 m (525 ft)||NSW, Zig Zag||40 km/h|
|1,435 mm (4 ft 8 1⁄2 in)||100 m (328 ft)||NSW, Batlow, New South Wales||Weight limit: 500 long tons (510 t; 560 short tons) and 300 m (984 ft) - restricted to NSW Z19 class 0-6-0 steam locomotives|
|1,067 mm (3 ft 6 in)||95 m (311.68 ft)||Newmarket, New Zealand||Extra heavy concrete sleepers |
|1,435 mm (4 ft 8 1⁄2 in)||85 m (279 ft)||Windberg Railway (de:Windbergbahn)||(between Freital-Birkigt and Dresden-Gittersee) - restrictions to wheelbase|
|1,435 mm (4 ft 8 1⁄2 in)||61 m (200 ft)||London Underground Central line||(between White City and Shepherd's Bush)|
|1,067 mm (3 ft 6 in)||60 m (197 ft)||Queensland Railways|
|762 mm (2 ft 6 in)||50 m (164 ft)||Matadi-Kinshasa Railway||original 762 mm (2 ft 6 in) line.|
|600 mm (1 ft 11 5⁄8 in)||50 m (164 ft)||Welsh Highland Railway|
|1,000 mm (3 ft 3 3⁄8 in)||45 m (148 ft)||Bernina Railway|
|600 mm (1 ft 11 5⁄8 in)||40 m (131 ft)||Welsh Highland Railway||on original line at Beddgelert|
|762 mm (2 ft 6 in)||40 m (131 ft)||Victorian Narrow Gauge||16 km/h or 10 mph on curves;
(32 km/h or 20 mph on straight)
|762 mm (2 ft 6 in)||37.47 m (122.9 ft)||Kalka-Shimla Railway||or 48 degrees|
|1,435 mm (4 ft 8 1⁄2 in)||29.00 m (95.14 ft)||New York Subway|||
|1,435 mm (4 ft 8 1⁄2 in)||27.43 m (90 ft)||Chicago 'L'|
|1,435 mm (4 ft 8 1⁄2 in)||25 m (82 ft)||Sydney steam tram
|Hauling 3 trailers|
|610 mm (2 ft)||21.2 m (70 ft)||Darjeeling Himalayan Railway||The sharpest curves were originally 13.7 m (45 ft) |
|610 mm (2 ft)||18.25 m (59.9 ft)||Matheran Hill Railway||1 in 20 (5%); 8 km/h or 5 mph on curve; 20 km/h or 12 mph on straight|
|1,495 mm (4 ft 10 7⁄8 in)||10.973 m (36.00 ft)||Toronto Streetcar System|
|1,067 mm (3 ft 6 in)||10.67 m (35 ft)||Taunton Tramway|
|1,435 mm (4 ft 8 1⁄2 in)||10.058 m (33.00 ft)||Boston Green Line|
|610 mm (2 ft)||4.9 m (16 ft)||Chicago Tunnel Company||6.1 m (20 ft) in grand unions. Not in use.|
|1,676 mm (5 ft 6 in)||175 m (574 ft)||Indian Railways||The sharpest curve permitted on Broad Gauge.|
|5 ft 2 1⁄2 in (1,588 mm)||28 ft (8.534 m) in yard,
50 ft (15.240 m) elsewhere
|Streetcars in New Orleans|
- Guide to Railcars, showing the minimum radii that each freight car is able to negotiate
- Canadian Light Rail Vehicle able to negotiate a 36 ft (10.973 m) radius curve
- Jane's World Railways 1995-1996 p728
- http://www.whatdotheyknow.com/request/24986/response/79568/attach/3/HS1%20Section%202%20Register%20of%20Infrastructure.pdf - page 19
- http://highspeedrail.dft.gov.uk/sites/highspeedrail.dft.gov.uk/files/hs2-route-engineering.pdf - page 4
- Australian Railway History September 2008, p291.
- Railway Gazette International March, 2012, page 23
- Railway Gazette International, July 2012, p18
- Trains: The Early Years, page 51, H. F. Ullmann,Getty Images, ISBN 978-3833-16183-4
- Lightrail now New Orleans RTA/Brookville streetcar