I have built my first extension to the layout, this is 68 x 3 inches so more of a "stick" than a "plank". It represents the continuation of the main line to "Fairport", a single line terminus located a bus ride short of Shelf International.
The nice thing about this arrangement is the train makes a "journey" along the plain line (no scenery) before it "arrives" at its destination. A second train can follow behind, and stop on the plain line at an imaginary intermediate location. And also, the whole layout now supports three passenger services instead of one: Fairport - Shelf (fiddle yard), Fairport - Creg, and the original route (with a switchback) Creg - Shelf.
The "baseboard" is a length of pine stripwood about 66 mm wide and 6 mm thick, braced with 25 x 11 mm stripwood. This is my first ever "all wood" baseboard (no man-made board), I have no idea whether it will stay flat but it is worth a try, if only to avoid buying a huge sheet of board and sawing off a silly strip. It is fixed together with nails and wood glue, the different pieces are all pulled into place to make the whole thing flat. This size wood is much easier to bend and hold into place than the traditional 2x1.
I filled over the nails with some knife stopper (my tub of wood filler had dried up), then I laid the track bed - this is foam strips by Woodland Scenics. I bought these strips instead of cork because I only wanted a small quantity but they are very good, I don't think I will ever choose cork again. The sound deadening is hugely better than cork and the width is more consistent. I glued the foam strips down onto the baseboard with ordinary PVA.
I screwed a length of alloy angle onto the end of the main layout to hold the extension, put the extension in place and then drilled a 2mm hole vertically upwards into the frame of the extension.
Then I took an old Lima steel axle and hammered it into the frame to make a dowel. The hammering flattened off the point so it is not sharp any more. So the extension has a positive location onto the layout, but it lifts out easily.
The track is two lengths of SMP, I glued these down with the 3M spray mount sold for photographs. I think this will hold forever, I did a test piece beforehand and the bond was very strong.
I put a rail break about 22 inches from the far end. This will hold a DMU at the terminus and let me run a train on the rest of the line. I have a personal convention of designating the far rail the "return" and the near rail the "feed" but this is a bit daft because it is much easier to hide track feeds behind the far rail. So I cut my break in the far rail this time.
I used a slitting disc to make the break, and then I slid the rail up to make the gap smaller, this way the break does not show so much. I don't use insulating rail joiners any more.
I put a sub-miniature toggle switch behind the rail break. This seems easier than wiring back to a central control panel, and it hardly shows. The 6mm baseboard top is too thick to hold the switch boss so I fixed the switch in with hot glue and finished off the top with a plain washer.
The extension is fixed in place with a wedge pushed in against the wall in the corner of the room. I managed to snag the rail at this end during construction and pull the rail out from some of the sleepers (Peco track is much stronger) so I had to solder in three copper clad sleepers to make the repair.
I have glued a length of aluminium angle along the front of the extension. This is partly for strength (68 inches between supports) but it is also to help the model blend in with the main baseboard beside it. I put some dark grey paint on the front of the wood above the aluminium, so the visible "baseboard" seems to be quite slender and floating above the aluminium.
The extension takes the total running length of the line up from around 10 to 15 feet, and I can now watch a train in motion for a minute or so from one end of the layout to the other.
Details of the station area at the end of the line are in a topic here.