PROTOTYPE DIMENSIONS

Before the model maker can begin construction of a layout, it is necessary to have some idea of the dimensions involved. The following gives major prototype dimensions which can then be scaled to the required size using the T.L.R.S. Standards.

TRACK

Curves: Real tramways had much larger curves than you think. The absolute minimum we have come across is for a 3ft 6in system and that was in the depot with severe speed restrictions. The curve radius was 35ft. In standard gauge 45ft was the absolute minimum. More modern tramcars need even more room. The absolute minimum radius for a Manchester Metrolink car is 25 metres or just over 85ft (The new Italian 'Sirio' tram is designed to run around 15 metre radius curves). On running routes the radii were much larger, for example the standard radius of a London Tramway point was 100ft! This is an area where modelling licence really comes in. There have been model layouts with curves which when scaled up to prototype would be 12ft radius!

Distance apart for straight tracks: Well on the prototype it all depended on the width of the road. The 'safe' distance was always allowing enough space between two passing trams for a pedestrian to stand without being touched. Taking the width of a tramcar at 7ft 3in (they varied a bit but this is the later wider cars width) then the track centres on straight track need to be 10ft 3in apart. On modern trams this will probably need to be 12ft to allow for the wider vehicles. If your layout has narrow roads, you may have to follow the prototype and allow only one car in the section at a time, or interlace the track or in extreme go to single track with passing loops.

Distance apart for curved tracks: This depends on the design of tram being run. On curves the 'swept path' becomes critical. The front and rear overhang makes the tram appear wider and do not forget that the centre of a long car will also sweep away from the track, but on the inside of the bend. Follow the prototype and design your curves to fit your vehicles. But even the prototype had problems. On the Hull system there were two curves where trams were banned from passing due to lack of clearance. When Hull 132 was restored the extensive and many repairs on the wooden dashes were evidence that this rule was often disregarded, to the detriment of the trams.

Groove: On older tramways the groove was 1.25in wide on the straight and 1.5in wide on tight curves. On modern tramways the groove has got bigger! It is now 1.75in on the straight. Generally the smaller scales have wider grooves, for example in 'OO' gauge a 1mm to 1.25mm groove is often used (a scale 3in to 3.75in).

OVERHEAD

Wire: The Inspectorate required tramway overhead to be no lower than 20ft above the road (at low bridges it could be lower by special authorisation). In fact most systems hung the wire at 21ft at the ears, this allowed for the dip of the wire between ears. Modern trams are under EEC regulation and the minimum height is now 5.2 metres (17ft) though Croydon has erected its wire so it is 5.8 metres (19 ft) high at the ears. The old wire was either of an '8' section with the ears clamping onto the grooves or it was a true circular section and the ears soldered or hammered onto it. The diameter of the circular section varied between 0.324in and 0.400in, the smallest wire practical was always chosen to save money, weight and the amount of fittings. Modern tramways have a double wire of inverse omega section, the upper grooves being used to clamp the wire in place. The wire is 11.7mm diameter (0.46in) and it is used double in order to carry the high currents required when the system is operating at full capacity.

Traction Standards: These hold up the wiring and come in three types, plain, bracket arm or double bracket arm, depending on the requirements of the situation. In addition in the city centres some wiring was hung from rosettes fixed to the walls of convenient buildings. The Board of Trade stipulated that the maximum distance apart for traction poles was 120ft. So this was the distance used by systems on the straight, on curves they were fitted closer to suit the situation. Here again modellers use licence and usually fit traction poles much closer (a scale 40ft apart) because it looks better on short layouts. The pole is usually fitted on the pavement by the curbside, rather like street lighting is today. However there must be at least 15in clearance between the traction pole and a passing tramcar. Traction poles were 31ft long of which 6ft was buried in the earth, leaving 25ft above the pavement. Older poles were of three sections, the bottom 17ft being about 8in diameter, the second 9ft 6in length being 7in diameter and the top 7ft 6in length being 6in diameter. Modern traction poles vary considerably, but all seem much bigger diameter. Croydon uses hefty 'H' section girder (Maybe they thought they were building a schwebebahn).

Traction Standards Bracket Arm: The Board of Trade did not set any limitations on bracket arms, but it was generally recognised that a bracket arm 16ft long was the maximum desirable. Beyond this span wires were recommended. Indeed later tramways had mainly span wires, particularly when increasing traffic meant the centre of the road traction poles had to be removed. The centre of the road bracket arm (double bracket) was popular in the early days. They were mounted centrally between the two tracks. As road traffic numbers and speed increased they were replaced with side bracket or more likely span wires.

Ears: The length of ears varied and the general sizes were 15in, 18in, 24in and 36in. On top of this there were splicing ears, feeder ears, pull offs and anchors. For the modeller the only pitfall is that the older, inverted 'U' type of ear can only be used for trolley pole operation and not with bows or pantographs.

These dimensions were compiled by David Voice.

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