Track Bed Design

What a track bed is for ? -

1. Transfer the load from the track to the ground

2. Hold the track in place

3. Allow for ground movement - stettlement/heave + sideways movement.

4. Drain away water + stop vegatation growth which can make the track bed unstable.

There is no standard design that fits all situations so it is worth looking at what the full scale railways do and learn from them.

Lessons from Standard Gauge

The above diagram shows a suggested make up for a standard gauge track bed. I have shown it here as it incorporates many features that you have to deal with in miniature railways.

• The minimum recommended ballast depth is 150mm. Given a standard sleeper is only 125mm thick ( railway sleeper is 250 x 125 x 2600mm) that scales to 30mm under the sleeper in 5" gauge.

• Geotextile fabric has been used to stop the sand blinding sinking in to the soil below. Apart from the sand blinding it is the same for 5" gauge.

• One of the most critical things is the different variations for dealing with welded and jointed track and thermal movement. This is often overlooked yet the same expansion rates apply whether it is standard or miniature railway. The sleepers are clearly entrenched in to the ballast to stop them moving. This should also be adopted in smaller gauges.

Geotextile Fabric

What is geotextile fabric

Geotextile fabric is a material used for road bases and has also found fame for use in weed control in gardening. Basically it is a material woven from a rot resistant plastic. The weave is fine enough to stop sand and plant growth going though but coarse enough to let water through. One of the trade names is Terram but there are several different manufacturers and grades. Make sure you use a heavy grade.

Do I need it?

YES

• Stops ballast sinking in to the soil/clay & stops the soil/clay ouzing up through the ballast

• Stops plant growth coming in to you ballast. Soil and plant growth in ballast breaks down its bond and so your track formation loses its strength

• Increases the load bearing strength of your track bed

• One method to provide a more stable sub base is to lay ballast, to say a depth of 30mm, on the geotextile fabric and then lay another layer of geotextile fabric. This forms a sandwich structure which has a greater bearing strength.

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A Suggested Track Formation for 5" Gauge

The above diagram is one suggestion for laying 5" gauge track in areas where the ground is fairly dry and stable.

As the ballast is not true scale and the forces being applied are different to full sized track, a ballast shoulder would be pretty in affective. On the above diagram and picture below, it can be seen that the block pavers are the key to stopping sideways movement of the sleepers.

Not only do the block pavers help keep the ballast in they also act as a boundary for the garden and grass. The use of thin timber boards means that the grass grows over and drops in the ballast. The concrete haunching under the blocks helps hold back the garden/grass. Where we laid the blocks straight on to the soil there has been a tendency for them to sink and tip over. The track in the above picture near the camera is slightly too low and in need of lifting.

Depth of ballast will depend on many factors -

• Ground conditions - load bearing capacity

• Track load - obviously 1ton trains require a stronger designed track bed

• Local conditions. e.g. Clay areas where the ground may heave as it takes in water during the winter and sink in the summer.round can change dramatically through the seasons.

• Heavy vegetation based soil that can sink and compress over time.

• Sandy silt based soil that can wash away with heavy rain.

While you may be inclined to dig down half a metre and bring it up with a compacted hardcore or crushed stone, you need to weigh up and decide if it is worth the effort. A track bed as illustrated above can still accommodate ground movement and if all it takes are a few re-adjustments a year to re settle the track is it worth moving tonnes of soil and other materials with no guarantee that the track still might not move.

Ballast

Is the name of the stone used around and underneath the track. Its main functions are -

• Distribute the load from the sleepers to the ground below

• Allow drainage of the track

• Prevent lateral and longitudinal (sideway and lengthways) movement of the track

• Provide an affordable material for packing and levelling track

Your choice of material is usually determined by what you can get hold of at a reasonable price. It wants to be a crushed graded stone and not rounded gravel based stones. The sharp edges of crushed stone interlock to form a better bed for transferring the track loads. We would suggest the stone wants to be graded to a maximum size of 20mm.

Some of our early Monmouth Green was of a smaller size and included much smaller pieces. While it locked together to form a solid base the smaller size meant that it held a lot more smaller soil particles that had strayed on to the track bed. Soil in the ballast encourages weed growth and reduces drainage which in turn can see sleepers wetter for longer which can lead to rotting.

Living in the suburbs of London our choice of material is limited and expensive. We initially had two 1 ton bags of Monmouth Green, from B&Q, but now buy top up smaller bags as and when. An interesting economic consideration is that a small bag of ballast, from B&Q, is nearly twice the price of the equivilent volume in concrete (without labour considerations).

On standard gauge lines track beds are often lifted around the 20 year mark and the ballast washed and re laid. This is to remove the smaller particles that can affect the ballsts performance. With a garden railway which borders flower beds the intrusion of soil can be more higher, so it may be worth following the fullsized practice. Where we have lifted track we have dug out ballast , washed and sieved it and it is surprising how much muck comes out.

Drainage

If your garden is susceptable to minor flooding or is generally damp all year round, it may be necessary to provide drainage. Introducing a ballast filled trough (the track bed) through damp ground will drain the surrounding ground. If your line is on an incline then you may experience ponding at the low point of the line.

The diagram above shows a track bed with a perforated plastic pipe below the track. The pipe can then be run to a low point such as a soak away or lower part of the garden. It shouldn't be run in to domestic drains as most water boards do not allow this.

To create a simple soakaway simply dig a large hole, line it with a polythene (make holes in it) or better still a geotextile fabric and then fill it up with hardcore to around six inches from the groound. Run the pipe in to the hardcore then cover with fabric/polythene and cover with soil. The gaps in the hardcore act as a store while the water soaks away.

Laying Track On Concrete

To some the ultimate may seem to lay track on concrete to avoid any future maintenance issues. On paper it may seem a good idea but it can be costly and still give you major problems.

Concrete track formations require a much deeper footing to remove soil layers that are prone to movement. One major problem with concrete is it isn't flexible. Large slabs can often sink or rise and create stepped joints. When slabs do move the only answer to adjusting the track is to pack it off with packers or break out the concrete. This operation is labour intensive compared with adjusting track in ballast.

Where concrete does pay off is in crossings and under track in station areas where you do not want any settlement.

If you do lay concrete - remember to allow for expansion joints. Failure to allow for thermal movement can lead to spetacular failures. It is not unheard of to have a slab suddenly pop up six inches or more.

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