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.