As long as you can reach the track and none of your locos or rolling stock hit the wall, there are no standards for track placement on your layout. There are suggested standards for centre-to-centre placement of double track, though, and the distance between double track on curves should also allow plenty of clearance for those parts of longer equipment that hang out well-past the edges of the roadbed. Passenger cars, autoracks, and articulated locos all may have excessive overhang at their ends on the outside of curves, and at their mid-points on the inside of curves.
While it's sometimes impossible, due to space constraints, to avoid placing track along the edge of the layout, there are reasons to avoid this situation if you're able: a major derailment may result in your prize loco ending up as a piece of scrap on the floor, and it's also very easy to bump things that are near the front edge of the layout as you reach over to get at something else. If you're interested in photographing your layout, it's always nice to be able to set the train into a scene, where there's at least a bit of foreground to add realism to the shot. If you absolutely have to place the track at the edge of the layout, try to place it so that it's not parallel to the edge, or introduce a slight, gentle curve: this makes the scene look less artificial, and de-emphasizes the fact that the tracks are right at the "edge of the world". At the back edge of the layout, it's much easier to hide things with view blocks and building flats, both in front of, and behind, the tracks.
Despite all that I've said on this topic, there are many areas on my layout where I've broken these "rules", usually to enhance operations, and usually caused by too much layout in too little space.
They don't photograph very well, so I don't usually show them.
The teamtrack siding at South Cayuga, added after the track and basic scenery was already installed: to add additional destinations for freight cars and as a possibe pocket track for holding a helper locomotive. The concrete floor is 44" below.
The shop complex at Lowbanks. Because the layout was so narrow, I didn't have room for a roundhouse, or the radial tracks normally used for access. In this photo, the handcars just visible at the right, and the loco immediately above them, are on two of the tracks into the shop building/enginehouse that I did manage to squeeze in. The track in the foreground provides access to the third shop track, plus to the caboose track. About 36" to the floor here.
The doubletracked mainline through Port Maitland isn't overly close to the edge on this section of the layout, which is only 20" deep, but I had to keep it mostly straight and parallel with the edge in order to accomodate the curves at either end. The angled sidings help to offset this design error. The siding right at the aisle was added for operational considerations, as this is an interchange point with the TH&B: any space for switching and train storage helps to keep this area from becoming too congested. It's only about 36" to "ground zero" here.
The siding for Hoffentoth Bros. icehouse, in Dunnville, is yet another locoation where the track parallels the layout edge. This was caused by trying to get some horizontal separation between the various grades on the mainlines running behind the area. Floor is a mere 40" away, as the reefer flies.
Not a very good view here, but the grade to my as-yet-unbuilt second level is on a broad curve right at the edge of the layout. The entire grade is about 45' long, laid out around several curves, with about 16' of it right at the brink. The maximum drop is just under 60". At least I followed the advice about adding a curve to make it look better.
Except for the last location, all of these design no-nos are on sidings, where speeds are low and the operator, with a walk-around controller, is right at-hand. The big curve on the grade will also be a low speed area: upgrade because the 2.8% ruling grade will beat most trains down to a crawl; and downgrade because it's downright scary when a heavy train starts to get away from the operator. The operator can walk along beside the train as it climbs up to eye level, part of the reason that this operational challenge was included.
Wayne
