Train Table standards are used by many North American LEGO train clubs for their tables. Clubs often (but not always) make their own tables. It is convenient to make tables an integral multiple of baseplates long in each direction. This results in tables that are not exactly an integral multiple of inches. (see the conversion table in the LDU article for why that is)
There are several different standards extant. One of the most common dimensionally is that started by PNLTC and further developed by MichLUG as seen on their page here. This standard uses 768mm (30.25") square tables (in baseplates: 3 by 3 32-stud baseplates or 2 by 2 large grey baseplates) or the extension to a rectangle, 3x6 baseplates or 768mm (30.25") by 1536mm (60.50"). You will hear these referred to as "30 by 30" or "30 x 60" as that's the nominal size in inches.
Trackdraw has common sized tables already included in its parts library for planning ease of use.