Track and Trackbed

Track

The track is Peco SM32 which has nickel silver rails on plastic sleepers. I just couldn't see myself making any progress at all if I had to assemble the track. Later I might try making the odd point (switch, turnout).

Trackbed Construction

I wanted to build a track which would be reasonably quick to build and could be removed if I change my mind about the route without leaving a substantial quantity of building materials to be moved.

The trackbed is constructed using wood splines which is rather different to the usual row of concrete blocks or planks on posts.

Initial inspiration for this form of construction came from this indoor railway, HO scale trackbed construction using splines and from this article about using recycled plastic 'lumber'.

I then realised that this form of construction is used on the raised track I run passenger-hauling locos on from time to time. In this case the longitudinal timbers are about 25mm by 100mm.

Track at the Guildford Model Engineering Society (3½" and 5" gauge)

 

My track is supported and located by two roofing battens on edge, running along the line of the track. The battens are fixed either side of a row of posts cut from 20mm galvanised steel electrical conduit, driven vertically into the ground on the centreline of the track. On the section I have done so far the posts are 600mm long, with most of their length in the ground. The soil is fairly sandy and loose with the occasional large tree root so hammering in the pins was easier than I expected. On the straight section the posts are at about 1m intervals. On curves the spacing is closer, down to 500mm. On the straights and very large radius curves the 19 x 38mm wood can be used as is. For tighter curves (certainly 2m and probably up to about 5m radius) the wood is sawn in half lengthwise to make two pieces about 8 x 38mm which can be easily bent to follow the desired line. The wood is fixed to the posts using wood screws driven through one batten, through a 19mm central packing piece and into the opposite batten, with some variation on curves. Using a screw either side of a post causes the battens to clamp up on the top of the post. The burr caused by hammering the post into the ground helps to stop the battens slipping.
 
I started with a straight section
 
The first section of trackbed under construction
 
The first section of track laid in postion to see how it looks
 
Trying out wood bending
 
Going round the bend
 
Clamped up ready for screws
 
Existing curve far right, work on new curve far left
 
Digging out the base of an old wall and some tree roots to make way for the first horseshoe curve at the end of the garden.
The batten is a measuring stick for the radius.
 
Eventually, I decided that it needed a retaining wall behind the track
 
Trackbed finished and ready for track. There's no soil in contact with the wood and my ground is fairly free draining.
 
The new wood was too light and showed through where the ballast was thin, so I toned it down a bit (and some extra preservative is probably a good thing).
 
The Peco track is held down by driving flat headed nails into the wood with the head overlapping the web of plastic between sleepers. Once the ballast is in place, the nails are hidden.

Ballast

I have seen photos of many garden railways where the ballast has let down an otherwise convincing railway scene. People often use whatever happens to be available at the local garden centre which is often too big, rounded (not angular) and the wrong colour. Current standard gauge ballast is 40 to 65mm cube which scales to 2 to 3.4mm cube. Recommended narrow gauge ballast is 25 to 44mm cube which scales to 1.3 to 2.3mm cube. Many lines seem to use smaller material.
 
There are several model railway suppliers of suitable ballast stone but they are all much too expensive in the quantities that I think I will need (total of perhaps a tonne?). I am now using Grano Dust, supplied in 25kg bags by Keyline Builders Merchants (ref 996250) at about £5 per bag. It starts off very "muddy" and I wash the very fine material out using a kitchen sieve.
 
Grano as supplied
 
After washing out the mud. It looks rather multicoloured here, but once in place I think it looks OK.
 
Newly ballasted track

Ballast sizes and much more:

Office of Rail Regulation

 

Track Laying

Expansion Gaps

Unless the track is very tightly constrained, to avoid snaking in hot weather it should be laid so that, at the maximum rail temperature, the rail section ends are just touching. As the rail cools from the maximum temperature, gaps between the ends of the rails will develop.
For a given temperature at which the rail is laid, the gap required will be:
Rail length x Coefficient of thermal expansion x (Max temp - Laying temp)
 
The rail is nickel silver, a copper alloy. I don't know the exact specification, but a quick search shows that the coefficient of thermal expansion of nickel silvers is between 16 and 17 x10-6 K-1. Let's use 17 x10-6 K-1.
The rail length is 1 yard, which is about 0.915m.
The maximum rail temperature might be 60C in the sunshine on a hot day. (If anyone knows different, please let me know.)
 
So, for example, if laying rail at 20C, the gap required will be 0.915 x 17 x10-6 x (60 - 20) = 0.62 x10-3m, ie about 0.6mm.
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