I’m running into a planning problem for my HO layout.
Do you plan your track around your “imaginary world” or lay the track plan and make your “world” fit?
A lot of both. First you should have your givens and druthers firmly established. What time frame, how much space can you take, what rolling items from which company(ies) will you use and what are their limitations as stated by the manufacturer (curves, tractive effort, etc.). Once you know how to limit yourself with your vision, mate that to the space you can occupy. Will you want/need staging? Where will it be on the layout? How will it be accessed?
Then doodle. Freehand what you must on paper, or commit a line diagramme to graph paper showing the work the railroad will do with distances between stops. Draw everything to scale. Once you appear to have something that could work, something that satisfies your bottom line criteria, map it out on the floor with masking tape that is 5/8"- 3/4" wide. Show your intended bench edges, too, and have a good look at what access and aisles will look like.
It is an iterative process for the vast majority of us. For those with simple ovals, it isn’t such a big deal.
Does that help?
Both. I try to start with a general idea of what the scene should look like when finished, and then try pencil in both tracks and buildings at the same time, playing around with both at the same time.
Then I change, modify and redo things over and over until I am reasonably happy with my plan.
And then I modify the plan again when I start building and see what works and what doesn’t work in real life 
A scene probably will look most realistic if buildings are not too “squeezed in” between the tracks, especially if you are not trying to create a cramped urban inner city/harbor like feel, but no matter what you do, you will have to make some trade-offs - very few people have enough space to make buildings the size they would be in real life.
Leaving e.g. enough space for roads and parking lots is often very hard to do if you are squeezed for space. Here is an example of a pretty good looking small switching layout plan (by forum regular poster Arjay1127) that shows leaving enough space between tracks for roads, parking lots etc:
Here is a link to a discussion of how to orient buildings and tracks on my current layout plan, showing how some of my plan was changed following a discussion in these forums about how to place buildings and tracks on my own layout plan to try to make a scene look reasonably realistic while at the same time being fun to switch:
Neither of the above - because I didn’t start with an imaginary anything.
My plan is based on something VERY real - the published timetable of a real rail line passing through some of the most beautiful countryside on the planet. The track plan is based on my observations of that rail line. Some selective compression was required, but only enough to make ten kilograms of real world fit into the five kilogram bag of my modeling space.
Once I had the track plan in schematic form, it was just a matter of looping the spaghetti to fit the ‘bowl’ of a practical (for my layout space) benchwork design. That wasn’t much of a compromise. The countryside I’m modeling is a steep-sided valley with a far-from-straight river at the bottom. The real-world locating engineers had to use a snake for a straightedge, and still needed a lot of bridges and tunnels to keep the curve radii (and speeds) reasonable.
This isn’t the first layout built to my scheme. I’ve been refining and polishing it for 44 years.
Chuck (Modeling Central Japan in September, 1964)
I would say it’s a combination of both. The way I did it is I made a rough track plan, and then decided where I could take out a siding or two to make room for a small town.
Thanks to all. All comments were a help. I’ll try to post as I progress.
Rich
Twin Creeks RR
I made mine work simply by having the train show up from a staging (Rest of world) and service a industry that is quite large and leave back towards the rest of the world. At some point in the future there will be a additio representing another place so a passenger train can come out of staging, visit the town and move on. That will complicate the industry previously built with that train length run around.
It is easier to build a little bit and make room for everything than it is to build alot of bits and try to solve the problem for everything else. That nice town would be no good if only a mountain mule was used to get to and from it.