It depends on the application. Do you mean for a double mainline, mainline siding, branch, industrial track, is the track curved or straight?
Set a box car down on the track and then move a box as close to the side of the car as you want. Remove the car and then measure the distance from the track to the box. Use that number.
Yes. 18" was the toy train industry norm for many years.
yes, see above.
Negotiating the curve is a different issue than remaining coupled to other cars while negotiating the curve. In general 20" will work for most all small equpiment. Small being less than 50 scale feet or shorter.
Not sure what you are switching from, but that doesn’t affect the answers I have for you…
Track spacing is variable, depending on yard or mainline or industrial sidings - or straight or curved trackage, etc.,etc. A typical spacing of 2 - 2 1/2 " usually works fine.
Distance from unloading docks depends, but most are close enough so the RR car opening is reachable from the warehouse, with sometimes a ramp being used.
Yes, you can use 40 ft cars and 4 axle locos on 20 inch radius trackage. If you do, I would certainly put in easements before going from straight tangent to a 20 inch radius. If there is a way to avoid 20 inch and go bigger, I certainly would.
I’m using 18-inch radius curves. I run 40-foot boxcars, 50-foot gons and 4-axle diesels just fine. Althouth the curves are far tighter than prototype, it works for me, and lets me get the railroad I want into the space I have.
Those were the days, ah yes, those were the days. The mid-60s for me, my teenage years, were the peak of “short-radius” railroading. The engines were short, those 4-axle diesels having replaced the steamers, and the cars were short, too, since big well-cars hadn’t been invented yet, and most freight was 50 feet long or less.
Go later, and your engines and cars grow in length. Go earlier, and the steam locos get bigger and bigger. No, the late transition era gives you the best opportunity to model with tight curves. As far as radius goes, it was the low point. Maybe that’s why so many of us are there.
Your proposed radius should be fine for that equipment.
A basic HO tool is suggested, an that is a NMRA track gauge. They are stamped out of stainless steel so one lasts forever. This little tool will provide you with the normal clearances for those loading docks as well as for any overhead obstructions. You will find yourself using it to check wheel and track gauge when you encounter problems with derailments. I’ve had mine 40+ years and it still gets used. This should be available at your local model RR hobby shop, and is an investment in good operation for the future.
First, go to the NMRA website here. They have published standards and recommended practices covering most of your questions. The NMRA stuff has been well researched and represents the experience of many years and many good model railroaders.
As far as track spacing is concerned, there are two issues. You need one spacing merely to permit trains to pass without hitting each other. This spacing is greater on curves than on the straight, and the sharper the curve the more you need. For yard tracks you also want enough spacing to allow fingers in to rerail cars without derailing cars on the adjacent tracks.
As far as loading dock clearance, if you don’t find an NMRA publication, use the NMRA gauge. It lets you check dock clearances, tunnel clearances, wheel gauge, coupler height, track gauge, turnout wing rail clearance. It’s a essential tool.
As far as curves, 18 inch radius is the standard radius in HO. All the trainsets are sold with 18 inch curved track pieces and so all the rolling stock makers go to extraordinary lengths to get their stuff to handle 18 inch curves. As a rule, all but the biggest stuff, 80 foot passenger cars, long well cars, and big steamers w