TG:Logic/Designations

Designations in the game are sections of track marked with the "designate" tool as being special in some way.

A short, preliminary set of designations would include:
 * Industry Loading/Unloading tracks (including loading-only, unloading-only)
 * Yard limits (single-tile track sections that mark yard boundaries; needs a good way to tell the game which side is yard and which is mainline)
 * Speed limits (two types: mark the complete track of a given speed limit, and mark only the ends. Again, needs a good way to tell the game which is which, in the latter case)
 * Inside yards, several specialized designations exist (and yard design is specialized enough to get its own article:
 * Engine/wagon idling tracks (further specializations: engines-only, wagons-only, specific wagon types only)
 * Fueling tracks
 * Sorting tracks
 * Arrival/departure tracks (including specialized A/D-only)
 * Lead tracks (usually only one per yard)
 * Repair tracks (can be specialized by building maintenance/repair sheds, speeding up progress on most operations)