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All About Clearances And Curves

Double Track Spacing.

Our railway builders fall into two camps, those who use rail-to-rail spacing and those who use centre-to-centre. The advantages of RTR really comes after the track has been laid, whilst CTC is better for the planning phase.

Our full-size railway in the UK has traditionally referred to the gap between adjacent tracks of a twin or multiple track section, as the “six foot”. We shouldn’t take this too literally however, but as a starting point it suggests the spacing between tracks centres is about 11 feet. In our scale this translates into around 151 mm. With typical stock no wider than 9 feet, this leaves a clear 2 feet between passing trains. There are several and varied reasons why we don’t use anything this tight on a G3 set-up.

Firstly, the rolling stock width permitted by the G3S is 140 mm, which equates to nearly 10 ½ feet lifesize. This overly-generous width is to allow for continental, but not quite US stock which is slightly over 10 ½ feet (128”).

Secondly our curves are much tighter than the average of our big brother. Bogie stock is caused to both project outwards on the outside of the curve, and underhang as a chord across the inside. Two such vehicles meeting on adjacent tracks of 151 mm centres, on any practical garden-sized curve, will have a dramatic coming together! I would hate to see this happen.

Thirdly, and relatively trivial compared with the above two, is the need for the ‘big hand in the sky’ to perform rescue operations in case of derailment. In a confined multi-track set-up there wouldn’t be much room for fingers.

The net result of all this is that the G3S standardise their portable track modules with 185 mm centres which can be downloaded here. This has been calculated to cope with the great majority of stock running down to fairly tight radius curves. As such it is a compromise, and so suitability should always be checked out, slowly and carefully by hand, if you bring your own stock to a strange railway!

Now we are not bound by any G3S police to use 185 centres; in fact we can use any centre spacing we like, as long as our own stock, and that of friends and GTG invitees, will circulate safely. Unpublished guidance from the G3S suggests that straight track could in fact use 160 mm centres, but that a transition is made along the straight leading to the curve such that the more appropriate spacing is reached as the curve starts. This does reflect full size practice where the "six foot" increases on curves. Historical investigations have been carried out by G3S on curves of various radii with a standard stock coach of length 79' 6" and 56' 6" bogie centres. It's now not clear why this length was chosen as it is much longer than a BR Mk1 coach (63' 6") and even the BR Mk3 (75' 5"), so the results are very much worst case. For the various radii the suggested track centres are given below. It is stressed that these are pessimistic figures for most stock, and one established GTG host currently uses approximately 175 mm centres on curves of 4.5 m radius!

Outer track radiusCentres (mm)
1.5 m258
3.0 m208
4.5 m192
6.0 m184
7.5 m180

On Apple Tree Railway I am playing tunes with the spacing, which will be down to 170 mm in many places on very gradual curves or straights. In theory this gives a spacing of 30 mm between the widest recommended G3 vehicles, or more like 50 mm clearance when using typical British stock. I am though creating gradual transitions to 180 mm as I approach curves of around 6 m radius.

Why do I bother deviating from the standard? Well I am confined for width in some places, such as in a cutting, over a bridge, and along some pre-cast concrete elevated track bed sections. These are all either straight or very nearly so, and so the overhang should not become a problem.

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Page last modified on February 25, 2018, at 07:56 PM