Editing Module:Track gauge/data/doc (section)

==About aliases and identifiers==
A rail track gauge is identified by "id". That id is the size in mm (a choice, made for this list). Preferably it has an article link where that gauge definition is described and sourced.
An entry is a data set (a complete subtable) with structure <code>{ ["id"] = "n", ... }</code>
An entry is identified by its Aliases. Aliases are unique; any alias will find only one entry (if present). Two entries can point to one gauge id: usually when definied in metric *and* defined in imperial. e.g., "1435mm" and "56.5in" are different entries (showing different primary size: in met or imp) but they point to the same gauge id: ["id"] = "1435".

The "id", "aliases", and "def1" fields are required. Also, some size in metric and in imperial must be present. That is: at least one metric size field (mm or m) and one imperial size field (ft, inch, or num+den) must exist. All other fields are optional.

:'''id''': An id stands for the physical size, a length. It can be reused in multiple entries, for example when a gauge is defined in both metric and in imperial. Even a third gauge name (by another alias) can use ths size-id.
:'''alias''' (input option): An ''alias'' must be unique over all data. Alias names here should not contain any spaces; spaces are trimmed from the input when it is processed, so alias names containing spaces will not be matched.