The railway has several levels. I constructed the main baseboards at the "zero altitude" line. The upper level sections have all then been added above that level.
The main "locations" are all level, but at different heights. (I realised they would need to be level to prevent trains running away down a slope). Modules for the fiddle yard, Clanfield Station, Petersfield Station, Portsmouth Station and Eastleigh depot were all made using conventional baseboard techniques so that they "sit" onto the lower level framework. In principle they could be removable, but that hasn't been intended.
Portsmouth & Eastleigh both have track running underneath them. To gain access, both areas are hinged so that they can be lifted up.
The sloping track sections that join the locations all have a timber support every 12" or so, attached to the lowerr level frame. The height of these was largely determined by trial and error, having set the height of the start and end of the slope and using a straight edge. The timber support is then cut to the exact length required using a mitre saw.
Once the modules for the locations were in place, and the timber supports for the track between them were complete, sections of trackbed (12mm ply) were cut to fit and attached using PVA glue and brad nails.
Most of the timber has come from "normal" DIY suppliers. This needs to be sourced locally, because of the bulk. Make sure it is straight: much of the timber available - even supposed constructional stuff - is warped and some would be good for building around corners. Store it flat, and preferably in the railway room for a few weeks before starting work.
Joints are glued and either screwed or nailed. Don't skimp here: it needs to stay solid through its life. Most of the screws were from Screwfix, as were the brad nails where used. PVA wood glue is readily available from DIY merchants in gallon containers, and it is much more cost effective in that quantity.
I used adjustable "feet" under all the legs. as constructed, the railway would be robust and stand flat outside the shed; it is attached to the wall at the very end to prevent accidents more than anything else. The feet, and the threaded inserts into which they screw were obtained from Ross Handling Limited for approximately 50 pence per leg. Leg part number 38SG-100; the matching threaded insert part number 250817.
The trackbed is 1/8" cork sheet, obtained from Quality Cork Supplies in Wednesbury (0121 567 3140). They advertise in Railway Modeller.