Space for your layout - how much do I need? First and most important advice: "grab" all the space you can.The absolute most common mistake is having your curves too tight:LGB GG-1 on 4 foot diameter curve.Even though you can find track that makes a complete circle in 5 or 4 feet (diameter), unless you want a "toy-like" layout with severe restrictions on what you can run, you need bigger curves.Just plain do not do the 4' diameter curves unless someone is holding a weapon to your head.Reasons for broader curves and using more space:The broader the curves, the more reliable the operation of your train is, less derailments.You can use larger locomotives on broader curves.You can run longer trains on broader curves.Long trains look funny on a short track.The the longer the mainline the longer it takes for a train to make one lap. There's very little fun in watching a train chase it's tail and come by every 30 seconds.Next issue: gradesThe next most common mistake is grades too steep. Mainlines rarely had over 1% and usually never over 2%. Logging railroads with geared locos maybe 5-7% or so.You need to measure to see what you think is not so steep. Try to keep your mainlines to 2% or maybe 3%Also remember percent is NOT the same as degrees. 3% is just 1.7 degrees. Be careful of what you are measuring!Here's an online calculator to convert degrees <> percent https://rechneronline.de/winkel/percent.phpThe Real WorldYou need to realize how "far" we are from the prototype in curves "sharpness" to see why sharp curves make unreliable operation.In the real world, curves are measured in degrees. Bob Hyman on MLS wrote:A 1° curve has a radius of 5729.65 feet. Curves of 1° or 2° are found on high-speed lines. A 6° curve, about the sharpest that would be generally found on a main line, has a radius of 955.37 feet. On early American railroads, some curves were as sharp as 400 ft radius, or 14.4°. Street railways have even sharper curves. The sharpest curve that can be negotiated by normal diesel locomotives is not less than 250 feet radius, or 23°. Even narrow gauge lines like the Rio Grande Southern had maximum curvatures of 24°, and there were only two curves that sharp … one at the Ophir Loop and the other at Trout Lake.To convert degree of curvature into an actual curve radius, simply take the sine of ½ the degree of curvature and divide it into 50 feet. The result will be the prototype radius in feet. To find the model radius, just divide the prototype radius by the model scale.A much simpler approximation is to divide the degree of curvature into 5729.65. The resulting number is the prototype radius in feet. Then divide that number by your scale and you have the model radius. Let's assume 1:29 scale. Even a 6° curve comes out to 32 foot radius. To us in the hobby, that is HUGE, and that represents a very broad curve. Conversely, a 5 foot radius is almost 40 degrees, which would never be found in the prototype, but is considered somewhat broad in our hobby.So, you need to broaden your curves all you can, and it will pay off in appearance, operation, reliability, and just plain enjoyment. When planning:Spend time thinking into the future. Yes, many people recommend that you just lay something down right away.Is there ANY time in the real world where doing a little planning is BAD? Resist the temptation, and look into the future. Don't just rush out and lay down a small oval, and then hope to expand on it later.It's a lot less fun tearing up an existing layout to move it around.Look for places to expand, you always need more room than you have.Look for ways to broaden the curves, places for passing sidings. Don't start with a small loop if you don't need to, look at where you will be in years to come, and see how to implement it in phases.Don't put the track where you cannot maintain it, avoid places that might have to be torn up, or where there is water drainage places.Don't do anything less than 10 foot diameter curves in a new layout.Spend the $99 for a track planning software, that lets you "snap together" pieces of track and do "what if" very easily, it's hard for anyone to visualize without some form of drawing.