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While planning my new layout, I would like to know what you feel the best selections of turnouts numbers and curve radius for a well balanced smooth running layout.
While considering going with hand built turnouts, there is a number of different turnout numbers, #4, #5 #6, etc. If you had the room and wanted a good trouble free operation would you go with say all #6 or #8 turnouts, and nothing less than a 22" radius curve?
The length of the cars make the biggest different in what radius you can use. If you keep your max car length to 50' then No# 6 switches are fine. I have tried running 30 50' car trains around 18" radius turns but found the 19" radius or more to be the magic number. I'm not sure why that is, but 19 seems to be it for long trains if the track is installed correctly. An 18" radius works fine for a 10 50' car train. Passenger cars and large freight cars generally need at least a 24" radius and a #8 switch. If your trying to run them on less than a 24" radius your looking for trouble. If your doing a 4X8 19 and 22 radius curves would be fine as long as the cars do not exceed 50 footers. The radiuses fit and you have a 3" space between the tracks.
If you have a traditional layout, #6 and #8s or greater would be the best. Avoid #4s unless you're in an industrial area or have a space problem in a location. Radius should be 24" or greater; you can get by with 22" if you have to. Again, if you're switching, you could go less.
On a switching layout, most would do the #4s and 18" radius, since the shelves they go on are space-challenged.
No, you would want to use the largest radius turnout you could. There's no reason to use smaller turnouts like #4's unless it's all the space you have.
In my yard I went with #4s in the ladder since that was what the space allowed. I can get my 72' passenger cars through it OK at slow speed. Truck or body mounted coupler setups can make a big difference as well.
The quality of the switch is a big factor too. I have a couple Atlas switches that will pick and and push the cars around like crazy - the frog just isn't right. I've tried to dremel it a bit to adjust the gage and it is better but not perfect by far. It seems that "you get what you pay for" is very true with switches.
The best are the hand laid ones since if you have a couple cars that don't (for instance) like the guard rail position, get out the soldering iron and do a slight adjustment. The more experience you get at making them the better they run.
I'm going to tag on to what Mike just said. I have no problem with #4 switches in yards, just don't switch with your Big Boy. Choose your turnouts and radius based upon function. Part of that is looks. When a big passenger car takes an 18" radius turn it hangs out on both ends and in the middle and looks more like a toy store ad than a railroad.
BNSF says #9 switches are good for only 15mph and is installing lots of 24s. Unfortunately, a crossover with 24s is several feet long in HO. Basically as big as you can with both radius and turnouts. I would consider #6 and 26 in my absolute minimum but I run modern equipment that doesn't look very good all cramped up. I also have a "heritage fleet" that is two brass steam engines that don't like sharp curves. I would go for less track and bigger curves if that was the choice.
Thanks for that list, Paul. It's kind of discouraging to realize my 48" radius curves on a former layout would still be considered "sharp" on the prototype. So, when I get my inheritance from my rich (but as yet unknown) uncle, I'll aim for a train room that will accommodate 90" radius curves. Let's see, we'll call that 12 feet for layout side clearance so that means a width of 24 feet. That'll just barely fit if I can find a good deal on a double-wide.
Since I'm running 89ft auto racks and 86ft hi-cube boxcars, my minimum mainline curve radius is 31", with #8 t/o's for my crossovers. This has served me well operationally, even though it doesn't look that great.
Somebody (John Armstrong I think?) once said, the length of straight track between curves on a crossover [or any other S-curve] should equal or exceed the wheelbase of the longest type of car you plan to run on your layout.
So, when I get my inheritance from my rich (but as yet unknown) uncle, I'll aim for a train room that will accommodate 90" radius curves. Let's see, we'll call that 12 feet for layout side clearance so that means a width of 24 feet. That'll just barely fit if I can find a good deal on a double-wide. 
