Track Help

Curve Tolerance

Written by David Head. Posted in Track Help

It's very simple really, your curves, be it a yard on a curve, a double track mainline etc; you have to allow a clearance factor so your trains do not hit when passing on a curve. Rather than finding out before making and laying your track, it is best to plan beforehand. To do this simply find the longest item of rolling stock that also has the longest overhang, ie a long wagon that also has its bogies well back, so when going around a curve, it swings out more. A long loco is also a good thing to try. A Big Boy articulated steam monster loco is another.

To test and ensure you do not sideswipe a train when passing, simply draw out a curve, the tightest you wish, then draw a parallel arc/curve at the next radius you propose to use. I used a stick screwed to a wood and a hole up the other end for the pencil. To change radius I drilled more holes to reposition the screw rather than the pencil. You then simply place your rolling stock onto the paper to compare and see the tolerances and clearances. It is good to have say three items of the same wagon, but you can mark the clearance points as well.

The pictures show me doing this very thing to test some ideas I have for out exhibition layout. The inner curve is 30 inches, our absolute minimum radius. On straights we use 45mm track centers. But for such a sharp curve we wan to allow a wider track center to allow for wagon and loco swing. In this test I set two track centers, 55mm & 65mm. I tested the closer tolerance first with my longest wagons - 80ft container wagons. - they passed with flying colours. Yes they could get a little closer, but we have a margin here for error and longer & wider locos/wagons.

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