A long route is a route that that comprises a series of two or more routes between consecutive signals.
E.g. Signal 1 has a route to Signal 2 which has a route to Signal 3. The long route is between Signal 1 and Signal 3 which is requested by clicking first on Signal 1 and then on Signal 3. This actually calls both routes sequentially and thereafter the routes and signals behave exactly as they would had they been called individually. Each route is requested sequentially so that if an individual route is not available, subsequent routes will not be requested.
This feature is not found very often and is only provided in the simulation where it replicates the real box.