look at subject
Typically I strive for zero gap, or close to it. Some of us allow tiny gaps to allow for expansion and contraction, but this would be along the lines of 1/32" ... a very small gap ... every 6 feet or so.
**Exception**: If you need an electrical gap, in which case, most of us glue in a tiny piece of plastic to KEEP the rails apart, electrically, filed down to the shape of the rail so trains roll smoothly across it. I personally use plastic rail joiners.
Brad
EMD - Every Model Different
ALCO - Always Leaking Coolant and Oil
CSX - Coal Spilling eXperts
First off, there is no such thing as a "Standard" gap size. It all depends on the conditions where a specific layout is located.
If someone could build a layout in a NASA clean room or equivalent, where temperature and humidity changes beyond 1% from optimum aren't allowed to happen, the rails could be butt-welded together and the only gaps would be electrical.
My own layout is at the opposite end of the spectrum. In the dawn hours of January, the temperature might be in the 20s (F) in my non-climate-controlled garage. August mid-afternoon typically tops out 100 degrees hotter. Humidity isn't an issue - it is usually in the single digits, and the benchwork is built of steel. I have found that if I lay tracks in the cool times I had better leave at least 1.5mm between rail ends. Anything less and the track buckles when the sauna heats up.
Most modelers will have conditions somewhere between these two extremes. If expansion is allowed for and doesn't take place, the wheels click at the joints. If expansion isn't allowed for and does take place, those nice tangent tracks will end up looking like sidewinder tracks. (Don't ask how I learned this...)
Chuck (Modeling Central Japan in September, 1964 - in a non-climate-controlled Nevada garage)
Many of us solder our curved segments when using flex track, and we leave several gaps along the tangents between them for reasons expressed above. Personally, I leave a 1-2mm gap, say a 1/16", every six to eight feet of rails, including just beyond the ends of curves that long....partway into the tangents. If you lay nice curves, leave them alone...don't cut them where they exit into tangent because the very ends of curves on flex will want to straighten.
Chuck, the above poster just now, urges us who use flex to get used to imparting permanent bends into the rails to help them want to stay in place. With some skill and care, you can even curve the very last two inches on each end of curved flex. Expect to pop some spikehead details though, while you learn what works and what won't.
Crandell
Gidday James, when laying flex track on timber bench work in a non climate controlled room with a temperature range from 23 F to 80 F, average 55 F, and an average humidity of 76%, I use an expired credit card to set my rail gap, that was after, like Chuck, I experienced "track buckle"
Cheers, the Bear.
"One difference between pessimists and optimists is that while pessimists are more often right, optimists have far more fun."