A Transportation Grid
Every transportation grid accommodates the lines of a particular transportation
mode such as the rail grid of a metro system: It is made up of a finite set of discrete
waypoints that are connected by tracks. In particular, a track that connects the
waypoint w1 to another waypoint w2 indicates the possibility of a vehicle movement
from w1 to w2; no more (such as a length, a curvature, or a slope) and no less. Thus,
every pair of a waypoint is connected by at maximum two tracks, namely one track
in each direction. Since by convention every waypoint moreover is either empty
or occupied by at maximum one vehicle at a time, then also note that conflicting
vehicle movements can occur iff two or more vehicles try to access the same waypoint
at the same time