The baseboard is the "table" upon which you build your Route. Expanding the Route or Map entails adding additional, adjacent baseboards (see).
The "Real Scale" dimension of one baseboard is 720 m (2,360 ft) or 0.72 km (0.447 mi) on a side. Thus, in your Trainz world, a baseboard presents just shy of 52 hectares or 138 acres of level land. The undeveloped baseboard is marked by grid lines that run north/south and east/west. Two grids are present: a yellow lines grid marking off 10 m by 10 m squares and a dark gray lines grid offset from the 10-m grid, dividing each 10-m grid square into four 5 m by 5 m squares. A single baseboard comprises 5,184 10-m grid squares with the center of each square the intersection of the dark grey lines marked by four small white dots. The default grid is the 10-m grid, but you can change this to utilize the 5-m grid, impacting primarily on painting ground textures and possibly performance (see).
For the most part, the grid needs not concern you in building your Route. However, several surveyor tools (adjust height and paint, for examples) only interact with the baseboard at the vertices of the grid. Thus, for example, clicking at a random place on the grid with one of the, adjust height tools at the smallest Cursor radius setting will only change the height if clicked on a grid vertex. Increasing the Cursor radius a small amount, so one or more vertices are encompassed, will result in the tool performing as expected.