Why grooved tramway rail?
An Englishman, C.L. Light, first devised a basic form of grooved tramway rail in 1856 for use with the horse tramways in Boston, Massachusetts USA. The design had the advantage over other tramway rail types of providing a surface flush with the surface of the road and not obstructing normal wheeled traffic, particularly horse drawn carriages with iron tyres, whilst still providing a reliable guide for the flanged wheels of the tramcar with the groove. Four years later, this design was modified by combining it with a form of the Vignoles (or standard T) rail, but there were production problems with rolling this revised design. These were resolved in 1878 under a patent held by Messrs Winby and Levick, and the perfected modern design of grooved tramway rail was first used in Nottingham (UK) the same year.
Another issue faced by street tramways is that they have much sharper curves than railways. This means that the flanged wheels on trams are more likely to bind on curves, and the flanges will ride up over the head of the rail, which derails the tram. The narrow lip or check on the grooved tramway rail acts as a restraint against this tendency by ensuring the flange stays in the groove, although this solution results in the characteristic squeal of metal rubbing against metal when a tram rounds a sharp curve. This means that rail wear of both the head and the check is heavier on curved track than on straight. Therefore the check on curved sections of track is often of a larger and higher cross-section than that of rail used on straight tramway track, in order to increase the interval between rail replacements. Even then, casual study of tramway rail on sharp curves will usually see evidence of heavy wear on both railhead and check for even relatively new rail.
The sharpness of tramway curves has a major effect on tramcar design, by limiting the maximum wheelbase achievable for both four-wheeled and bogie tramcars. The longer the wheelbase, the more likely a tramcar is to derail when rounding a sharp curve. However, the shorter the wheelbase, the more likely the tram is to hunt or yaw from side to side sometimes this oscillation mode can be quite violent on four-wheeled tramcars. The Brill Radiax track as used on the Melbourne T class and Geelong Pengelley trams was a not altogether successful attempt to address this issue, by using a long wheelbase truck (12 feet as opposed to the more usual 6 to 8 feet) which forced the axles to take up a radial orientation when rounding a curve, reducing the amount of wheel binding around the curve.
The advantage of the bogie tramcar is that the tram body is isolated to a large extent from track irregularities, as well as dampening yaw mode oscillation by allowing relative freedom of movement of the bogie. This means that bogie tramcars give a smoother ride and have the potential to reach higher speeds than four-wheeled trams.
The ride quality of tramcars using Radiax trucks is almost as good as bogie tramcars, but the complication of the truck design and increased number of moving parts leads to more frequent service intervals and greater maintenance costs than both standard four-wheeled truck design and bogie design cars.
In some respects the advent of modern low-floored trams such as the Combino and Citadis are a retrograde step back to the obsolete four wheeled tramcar designs, due to the axle-less design constraining the isolation of the body from the movement of the wheel-sets. The leading body unit of both of these tramcar designs has a marked tendency to hunt at higher speeds, and the ride quality is quite harsh in comparison to older bogie tramcar designs.
As a result of these design issues, use of bogie tramcars reduces wear on the rails, particularly on curves, over four-wheeled and modern low-floor designs although not all bogie designs are created equal. The bogies fitted to Melbourne Z1 and Z2 class tramcars are notorious for chewing out curves which led to the selection of a different bogie design for the follow-on Z3, A and B designs.
Content copyright © Russell Jones 2001-4. Reproduced with permission.