Due to the nature of the site I have been working on completing as far as possible the short leg of the "L" out in the main room, prior to pushing the whole unit into place above the stairs. This section is effectively a diorama, designed around a trestle-bridge inspired by the CPRs "Half-moon Trestle".

Above is the view down the staircase. The "storage-bench" to the right will be the long leg of the "L", and the area straight ahead with a jumble of paint-tins etc. will be the short leg. Just visible top right is the unit which will (I hope!) slide across to the left.

Above is a closer view of the short-leg unit as it exists today. I thought I needed to complete the landscaping as far as possible before working on the trestle itself - due to the need to work around and behind where the trestle will be. The track across the canyon is supported by the base foam that was cut out from beneath the track in making the valley, held in place by blue electrician's tape. (Note the slanted ends, making it possible to rest those ends on a firm base at either end of the gap.)

Above an even closer view of the crucial area. Note the short length of trestle-base lying on top of the foam which is (so far) the only casualty in my process. (The base was built and glued to the bottom of the track before the supporting foam was strapped in place.) The black line you can see along the top of the foam is just that, the marking for the track-centre as originally traced onto the foam base when it was whole (i.e. this is just one piece of foam, not two side-by-side). The foam-supported track has taken quite a few knocks in the landscaping process, so I'm grateful to have just the one casualty to glue back in place in due course.
Progress on the layout so far has, I admit, been slow... (I told my wife it was a ten-year project, and she keeps asking when that started from.) Which is why I have leapt at the chance to use this "Challenge" as a spur to get things moving. I've just about convinced myself that there is no more tweaking to do to the surrounding landscape (there may be more trees to add in front of the trestle once it's built), so the building of the trestle itself is the focus for this log. As emphasised in my second sentence I am a first-timer at all of this, so any helpful comments and tips as I go along will be very much welcomed. Here goes...