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Minimum radius

In narrow gauge recommendations - TWO FOOT GAUGE

  • Minimum radius should be 20 times the longest wheel base. The absolute minimum is 12 times the longest wheel base.

  • Gauge widening of 7mm for curves between the 20 times and 12 time wheel base. Equivalent of 1.5mm in 5" gauge.

5" Gauge Example - Trojan wheel base is 254mm

  • minimum radius would be 5.08m

  • absolute minimum 3.048m (10 feet).

  • Any more gauge widening and the bogies can crab, leading to the bogie turning out of the curve and increasing friction and wear on the leading wheels.

That is the theory -

In practice if going down to a six foot radius gets you a loop or return on the line then it is worth doing.

Picture - Philip Moroz

The line above is the Wooburn Green Railway. Six foor radius and one very proud owner. Power is provided by a Trojan and trains are short. Don't let the theory stop you from building a railway.

The tracks in our tunnel - inside is a 10 foot radius.

For full sized railways the practice of going from one curve straight in to another should be avoided as it can lead to buffer locking. Most miniature railways use a bar connector so buffer lock shouldn't be a problem. That said excessive direction change is uncomfortable for passengers which can lead to derailments.

0-6-0 engines - Having a centre set of wheels can cause problems. If you are intending to run an 0-6-0 on tight radius track it may be worth removing the flange from the centre set of wheels. We have done this on our electric Thomas and it means it can now traverse the inner loop. It is a brave one way decision but the benefits may pay off.


It is very hard to track down figures on gradients. There are numerous factors that make most rules and formulars a waste of time.

Factors affecting gradients

  • Lead in to the gradient - standing start or run up
  • Type of rail
  • Weather - wet rail can see loss of adhesion by over 50%
  • Dirt oil on track - as above
  • Load - 1 or 20 people
  • Type of engine - Hercules will take a 1 in 20 many won't
  • Curved or straight?
  • Rolling stock - huge variation on friction/resistance of different designs

Some of our tests

Clark at Harrogate 2004 1 in 10

Hercules on a angle iron track. Pulled 18 stone adult from standing start.

1 in 20

Hercules on steel track, 6.5 feet radius. Two adults from standing start. Could have pulled more but no volunteers.

1 in 40

Chingford Various gradients - 9 adults after a days running. The engine was more than comfortable pulling this load around the track.

A couple of things to think about

  • Avoid gradients in sidings and stations if at all possible.

  • On our railways the 10/12' radius curves and steep inclines means most steam engines are limited to the driver + 1 passenger.

  • Walking up a 100mm rise is barely noticeable over a distance of 2 metres. The same can no be said for trains. Move the landscape in preference to the line when building a line.

Next Stage - Site Survey

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