BRIO track geometry: why some loops close and others nearly do
BRIO track, and the wooden track made to fit it, is built on a 45 degree curve: eight to a circle. The straights come in four lengths, all multiples of 18 mm. The geometry looks simple and is not, because a 45 degree curve advances the line by 101√2 mm, which is never a whole number of 18s. That is why so many BRIO layouts almost close, and why the joints are made with a little play.
The pieces
| Piece | Measurement |
|---|---|
| Mini straight A2 | 54 mm |
| Short straight A1 | 108 mm |
| Medium straight A | 144 mm |
| Long straight D | 216 mm |
| Large curve E | 45 degrees, 202 mm centreline radius (182 mm inside, 222 mm outside) |
| Short curve E1 | 45 degrees, 110 mm centreline radius |
| Curved switch L and M | a medium straight with a large curve grafted on at the same end |
| Short curved switch | a short straight with a short curve |
| Cross track | two 116 mm lines crossing at right angles |
| Ascending track N | 216 mm, rising 64 mm |
| Bridge spans | tied arch 144 mm, arch 216 mm, suspension 360 mm |
The Vario System
BRIO’s pegs are smaller than its holes, on purpose. Each joint can be stretched by two or three millimetres and bent by a few degrees, and the company calls this the Vario System. The guide on woodenrailway.info shows that even BRIO’s own square-table double loop misses by about 2.7 mm at one joint and calls it a fit. The planner allows 3 mm at a joint, the top of the published range, and says on every plan how close the join is.
What a switch replaces
A curved switch is a medium straight and a large curve joined at one end, so it can stand in for a medium straight anywhere in a loop and leave a curve free for a siding. The short curved switch does the same for a short straight and a short curve. Because BRIO track is grooved on both faces, any switch can be turned over to become the other hand.
Why the planner sometimes says no
A DUPLO figure of eight is exact. A BRIO one is not: the two lobes want to meet 5 mm apart, more than a joint will take, which is why the classic BRIO figure of eight is sold with a viaduct that hides the difference. When the planner leaves a shape out, it is because the arithmetic says the ends do not meet, not because it has not thought of it.