Track Construction & Standards

3 ½", 5" & 7 ¼" Gauge Track Notes

Introduction

The rising popularity of ground level track has introduced problems not generally associated with the more traditional raised track systems.

With a traditional raised system normal turnouts ( points ) and other complicated trackwork are virtually impossible and the problems associated with them are consequently of little or no importance. The only variation is a slight increase of the gauge on curves. The so called " gauge widening " Usually never more than 3/32" and on large radius curves may be virtually disregarded. The notes below apply almost exclusively to track having pointwork etc.

When considering the building of a ground level system with the attendant crossovers, switches etc. we need to look very carefully at the standards that we propose to adopt and how we implement and enforce them. There is little point in superb track built to a set of standards that no club Locomotive will run on, or to a set of standards that will not allow visitors Locomotive's to run either. We must first establish what the most common club back to back wheelset dimension is and the flange profiles. For the sake of reliable safe running some members may need to modify wheel profiles or even re-wheel!! The recent requirements of the HSE and your risk assessment make this a must.

One of the main causes of derailments is incorrect back to back dimensions. It is a good idea to publish these dimension and tolerances on your club web page, this will also enable potential visitors to check the suitability of Locomotives before making expensive and possibility wasted journeys to a strange site. Do not fall into the trap of opening up check gaps to cater for the odd rogue Locomotive. It will only lead to disaster with regular annoying derailments, or even worse. Set your standards and work to them. Be wary of producing a new club " non standard" standard, we have enough standards already !

Back to Back Dimension

It is true to say that the wheel set back to back dimension is the single most important dimension in track design. Other dimensions are critical but they all stem from the back to back dimension, and of course the gauge. An accurate back to back dimension and a good flange profile will go a long way to ensure reliable running. Do not forget to cone the wheel treads, this provides a self centering effect and automatically corrects for wheels of different diameters on the same fixed axle, within reason !!It should always be noted that ground level stock is more prone to derailment as the center of gravity is higher. A large steam locomotive and stock coming off on a curve when full of passengers is not a pretty site. It can also be expensive as members of the public are very keen to litigate these days, even if they are being carried free.

Turnout Construction

It is theoretically impossible to build accurate multi-gauge track, since one rail is always a common carrier. As the rail section and the check gaps are different for the various gauges, clearly we have a problem, as we cannot have two sets of standards on one rail. The fact that it is impossible does not seem to matter much in practice!! As usual the answer lies in a compromise. We do however need to manufacture pointwork to very high standards of accuracy and ensure that they cannot move out of gauge in service. Fixing with nails or screws to any old piece of timber is not acceptable. A welded construction or the use of proper chairs etc is essential, made off site, to proper engineering drawings and built up in a jig. Both Dennis Monk and Martin Baker's articles cover this in great depth.Successful pointwork has been built to run all three gauges together. This can only be done by the use of 4 sets of running rails. See attached drawings.

Transition Curves

One factor that is nearly always neglected in Railway Track Construction is the provision of Transition Curves. As the Locomotive enters a curve it does so gently and with an increasing degree of superelevation. If you go to most model railway exhibitions the Locomotives seem to lurch into the curve which usually goes from a perfectly straight piece of track to 3 foot radius in zero distance. Superelevation ensure that the centrifugal force of the stock is transferred vertically into the track bed without imparting any radial loads. Radial loads impart stress into the wheel flanges and cause lateral movement of the track. A vertical load goes straight down the rail into the sleeper and ballast with, ideally, very low or zero radial loads. Superelevation is a bit hit and miss as it is a function of vehicles angular velocity, for a given radius, and not all trains run a the same velocity. Here again a compromise is required. It is true to say that for model use at low speeds it is largely cosmetic. This subject is very fully covered in Dennis Monks article.

Rail Sections

Rail is available in two materials Aluminium and Steel. Both have disadvantages and advantages. As for these materials, Steel looks right, wears well especially at joints and gives good adhesion. For a given load smaller sections can be used. It is also easier to weld. The only disadvantage is the rust. If track is used infrequently it can get stock very dirty. Alloy is clean, but tends to be " greasy " rail joints not supported well can wear, leading to rough running. Rectification is difficult. Alloy gives a softer ride whilst steel tends to be harder. On balance Alloy is probably more suited to raised track whilst Steel seems better at ground level. The modern approach is to use correct Flat Bottom rolled section, although many tracks use Mild Steel Flat Bar without any problems. This type of track material lends itself well to a fully welded system.

Rail Cant

Reference to any full size track information will show that, at least as far as Bullhead rail is concerned, the rail section is canted inwards by the design of the Rail chair. This has the effect of presenting to the wheel profile a different form than if the rail were truly vertical. Flat bottom rail is rolled " symetrical " but the form on the running head is different to that of Bullhead section. Rail cant is universally ignored in the smaller gauges.

Published Information

The best published information on Model Track Laying to date was produced in " Model Engineer " some years ago. The series of articles were entitled " Making Tracks " by Dennis Monk, Model Engineer 19th August 1988 to 16th March 1990 in 20 parts. This is a real tour de force on the subject and is a must for anyone building model ground level trackwork in the larger gauges. It covers all aspects of track construction from site preparation right through to wheel flange dimensions, safety etc. Other articles on relevant subjects have been published in Engineering in Miniature from time to time, in particular by Martin Baker.

See Further Reading below.

For a table giving dimensions for various Standards click on :-

3 ½" Standards Information or 5" Standards Information or 7 ¼" Standards Information

Further Reading:-