A railroad route is identified by an origin, an optional sequence of intermediate points, a destination, and the railroads that connect them. There are two types of routes: single-line and inter-line.
Single-line routes include an origin location, a destination location, and a single rail carrier serving them both. Inter-line routes occur when different railroads serve the origin and destination points, or another railroad must be used to "bridge" the origin and destination carriers. Inter-line routes require one or more junctions.
Between any two rail points in North America, there are a large number of possible routes (traffic between Atlanta and Dallas could go via New Orleans, Memphis, or conceivably even via Chicago). Realistically, there are a limited number of feasible or practical routings based on which railroads serve the points, their operations, and the junctions at which they interchange cars.
When the same railroad serves both the origin and destination, a single-line route is usually most practical. While in some cases inter-line routes may be valid alternatives to competing single-line routes, they require coordinated effort between multiple railroads to interchange traffic and share the responsibility/revenue of the shipment. In other cases, no single-line route exists and one or more inter-line routes are the only way to get between the origin and destination.
In the U.S. and Canada, there are about 7,500 route-miles where railroads operate primarily in one direction over a rail line, with traffic in the opposite direction moving over an alternate route. These directional routes are reflected in PC*MILER|Rail’s routing and mapping.
This map shows how southbound UP traffic from Chicago to Houston moves via Shreveport, LA, whereas the northbound traffic moves via Little Rock, AR.