What is the correct spacing to use between two parallel tracks in OO gauge?

The distance between real parallel railway tracks can vary depending on the line speed, when the track was installed and any infrastructure encountered along the route. If recreating a real location, it is up to you how exacting you are with your parallel track spacing. Some will scale the prototype's track plan down to the nearest millimetre, while others will take the essence of the prototype and make it fit. There is no real right or wrong answer here, just so long as trains on opposing tracks don't collide (especially on bends) and the layout looks good. But what spacing should be used for a generic model railway?

When planning a OO gauge model railway that is either generic in design or a fictional location, deciding how far apart to place parallel tracks can be a bit of a head-scratcher. Of course, you are free to design your railway the way you wish subject to the above two rules, but your layout will come together much easier if you let your point work dictate the spacing between parallel lines.

When it comes to points (turnouts), there are two main types available, regular or streamlined. Regular points, such as those found in many starter sets, will naturally produce a distance of 67mm between the track centres of parallel lines. That spacing is a little wide compared to many real-life railways, but it is also necessary to provide clearance for trains passing on 1st, 2nd or 3rd radius curves. If you desire something a little closer to the real thing, Peco's Streamline points provide a more realistic solution with track centres just 51mm apart. These types of points are more commonly used in conjunction with flexible track and care should be taken when designing bends that long coaches or rolling stock can pass without colliding.