Ballast Size

One of the questions which is often raised in model railway fora is: "what size of ballast should we use ?"

The answer depends on the scaling of the prototype relative to the scale being modelled.

Our observation has been that commercially available model ballast products are usually overscale for the scale they are sold for. Typically, we find that 4mm scale (00) ballast is best suited for 7mm scale (O gauge) and 2mm scale (N) ballast is better suited for 4mm scale etc.

What size is prototype ballast ?

On Britain's railways, prototype ballast is a grade of different stone sizes, ranging from half an inch (12.7mm) to 2 inches (50.8mm), evenly graded.
The reason a mixture of sizes is used is so that the stones lock against sleepers and each other to hold track in place without movement.

What size are model ballast products ?

We measured some commercially available products as follows:

Product Average Stone Size
Chuck's Marulan Fine 0.71mm
Heki HE3329 0.62mm
Woodlands Fine 0.52mm
Woodlands Medium 0.82mm

What size ballast should we use for different scales ?

The following table shows the appropriate stone sizes for different modelling scales:

Scale Scaled Size (mm) Suitable Product
7mm 0.29 - 1.16 Chuck's Marulan Fine, Heki HE3329, Woodlands Fine, Woodlands Medium
4mm 0.16 - 0.66 Heki HE3329, Woodlands Fine
2mm 0.08 - 0.32

The "Scaled Size" column indicates the correct range of sizes appropriate for the corresponding scale.
Of the selected products, all could be used for 7mm scale, however, they would need to be mixed with a larger stone such as Woodlands Large. All are too large for 2mm scale.

Stone Types

Granite was generally the preferred ballast product, however, the Southern region had very serious problems with granite grinding away at concrete sleepers which exposed the aggregate abd caused crumbling. The Southern used carboniferous limestone, mostly from Westbury quarries, which is anything but soft. It is also much cheaper than granite. The Southern favoured granite under timber sleepers because it has sharper edges than carboniferous limestone to stabilise the track. Generally they had to put up with limestone because granite was too expensive.

Graham Plowman (Updated 26/05/2019)

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