My Approach to the Problem

Had the same realizations as you have.

Track plan designed with XTrackCAD until G&D list satisfied. Then roughed in major scenic elements, buildings, rivers, bridges, and roads in scale (close) using XTrackCAD shapes. Based dimensions on prototype in some cases, dimensional data from Walthers site in others. Final plan am building from.

Yet to cut plywood so we will see how this goes.

Design Detail

Well maybe.  I start with a rough idea of where the mainline and major features need to be (staging, yards, significant scenes), and refine their positions before building so I don't have to re-work too much as I go.  Finished locations of industry tracks may end up moving a few inches here, or a few feet there.  I can assure you those big published plans almost invariably end up changing if they are built.

Often I will build from a location I fully understand toward one I don't, and figure things out along the way.  One such spot was described in my blog a year ago, where I needed to halt mainline construction while I kitbashed an industry, as I was working in a narrow area and wanted to know the exact size and shape of the industry before locating the main track.  In another, I held things up while I built a steel trestle.  In still others, track had to go where it had to go to accommodate curves and turnouts, so scenery and structures had to move accordingly.

It's easy to let rigid constraints get in the way of moving from design to construction.  As you plan, you will see where there are critical locations where the design must be exact, so get those ironed out first (a structure you want to use, a crossover or siding that must be in a particular spot, etc.). If there's wiggle room elsewhere, allow yourself to take advantage of it as you build.  If there are places where you think you might want to make changes, build so you can incorporate those without too much aggravation (such as to leave access to screws so you can move a joist).

Another example

Rob brings up some good points. I did some similar work on my previous layout. I did not have mountains or canyons to deal with but I had a limited space and intricate trackage that needed to fit. After making my plan I plotted tangent track centerlines onto the subroadbed, then added the curved portions and easements to join curves and tangents. Along the way I would place track, turnouts, rolling stock and building mock ups to ensure the final version would function as needed. I featured this in a December blog post here:

http://designbuildop.hansmanns.org/2011/12/08/hitting-the-fan/

Granted, i was working with a small layout area of 11x11 feet. On a larger layout a similar exercise can be done at each town or industrial area. But it can be done even before cutting the subroadbed.

Eric

Track first

I designed the track first.  All track, main, sidings, yards and industries.

With a few exceptions the track can be adjusted a couple inches one way or the other.  So once the basic design was set I then tweaked a couple places to make a little more room anticipating a building or scenic feature. Then I cut the roadbed.

I rank things track first, buildings second, scenery last.  Since I have narrow benchwork to fit in the room, typical depth 18", the vast majority of my builidngs are "low relief" type that intersect the plane of the backdrop or fascia.  I have fewer buildings that have all 4 sides.  So I have no problem whatsoever slicing and dicing buildings to make them fit whatever footprint I need. 

Since scenery is the least impacted by relative size, if anything has to take it on the chin, scenery does.  I make it fit in the space that's left.

Having said that, if there is something I can redesign on the track that makes a big improvement in the scenery, then I don't have a problem making those changes either.  I had a continuous running connection on my layout, but it cause a lot of issues compressing an industry, impacting the track and scenery of a station on an level above it, and complicating the wiring of a wye.  I eventually decided to remove the connection and simplify everything.

Avoiding joists - supporting terrain

By the use of risers, much of the terrain can be formed with little concern for the benchwork joists or cross-members. The risers need to be high enough to provide for the negative elevation of gorges, draws, creeks, rivers, lakes, et al. As an example, look at some of the construction methods used by Rick Wade (rtw3rd) in his alcove expansion blogs here. The MRH Mag parent organization, Model Trains Video, has videos that explain this method. The roadbed can be either spline or "cookie cutter" segments.

Ah yes, joist location

Bill's comment reminded me of a step I failed to include in my first reply - joist location.

To solve this issue my benchwork will initially be built on top of a 1:1 print of the track plan before being placed on wall brackets.

The value of risers

All at once

I prefer to design a layout all at once. That is while I'm considering the location of track, turnouts, yards, spurs, etc. I also try to plan how the scenery/structures will work around it. Generally I don't worry much about how to build it at this point.

What does concern me is whether it's feasible to develop scenery around the track to justify it's twists and turns. Real railroads don't do curves in their track just because they look kind of nice. Curves are used to get track to connect somewhere, avoid a natural obstacle (hills, ravines, excessive grades, etc), avoid a man made obstacle (property line, structure, etc).

I also try to think in 3D. That is if there are two tracks (lines) on the plan next to each other, I don't want them to be more than an inch or so vertically separated (for the most part). It's hard to scenically justify two tracks, 3" from each other running parallel for 12' where one is 8" higher than another (unless you really enjoy modeling retaining walls).

It's also a time to consider whether highway's passing under track, or the bottoms of rivers will clear any hidden track (staging area or a lower deck) that might be running below them. Whether the tops of mountains (or a skyscraper) will clear what ever is above them (the ceiling or the bottom of an upper deck).

Another place to check vertical distances is at tunnels. If you're planning a tunnel, but the hill being "burrowed" is only 4" or 5" tall (in HO) most likely that track would be in a cut instead of a tunnel. When planning a cut, check the distance from the track clearance points to any place on the hill that can't be cut away. Railroads have civil engineering standards for how much level space must be next to the track before the sides of the cut slope upward to the top of the hillside. The grades of the cut's walls are also specified in civil engineering railroad standards and will vary depending on the type of material modeled in the cut walls.

Finally, I check that all track above track situations (one track passes over another on a bridge) have sufficient clearance.

Planning everything at once in a 3D (not pancake flat railroad and terrain) layout will pay big dividends in having a more believable layout/scenery situation. After the track is laid is NOT the time to start on your scenery plan if you're going 3D. And even Kansas isn't completely flat.

Best regards,

Charlie

Last layout I built.....

After getting to the point of having a passable layout plan I made a photocopy and turned it into a 3D scale model about 1/10 of the real thing. Made a few wood blocks out of balsa to represent cars and locos and major buildings (not scale models, just something as close as possible dimension-wise). Needless to say, major revision followed due to unexpected vertical clearance issues. Total time spent: a few hours. But the aggravation of not having to do major rebuilds was worth it.

Vertical clearance and CAD

Vertical clearance checking in another place a decent CAD program can shine. If you're rigorous about setting track elevations the program will tell you what sort of grade you're facing. I check vertical clearance in 3rd Planit by setting the camera to follow the track at 3.5" above the rails. Then you can drive the camera around the layout in 3D mode as if you were in a loco cab. If I see another piece of roadbed coming through my forehead, I know I've got a problem.

Charlie

good advice so far

Thanks!   Interesting and useful comments (and as a first-time poster, much appreciated!).  

As everything else in this hobby, I see some variation in approaches.  That in itself is a useful tidbit...ie., there's no one 'right' way that people plan their layouts either.

I'm a software developer, so throughout my career, my creations have "fit" without physical constraints - it's all abstract data structures and such - and suddenly I find myself impaired by having to deal with reality in the form of altitude and clearances!

So I've used the CAD approach extensively, and couldn't do without it. Fantastic tool. My problem is more in a lack of knowledge of exactly where to place those CAD-precise lines due to the "other" as-yet-undefined constraints, so I'm going back to find some building footprint drawings as one poster suggested and that'll probably help some. The 3D model is also a great idea that I had forgotten about, and is probably worth doing for a final reality check.

The other important thing pointed out, and I keep forgetting about, is that all the precision of my planning will probably need adjustment anyway when I swing the hammer and sling the plaster!

Overall, BearCreek's holistic approach is what I'm striving for (and having trouble with), and being purely 'freelance' is making my scenery planning harder for me since I don't have a specific prototype to catch hints from. Fortunately, I have a long list of "druthers" (don't we all) to pick from that suggest scenic treatment, and I've had some small success already by thinking about smaller independent segments of the layout (as suggested by another) within Charlie's kind of 'believable' context, instead of the whole layout at once.  Divide and conquer...

Thanks so much for the comments so far, I hope to hear more on this.

JimW