Description of single track ABS system used on the Southern Pacific . Proceeding on signal indication is safe, but not necessarily wise. It is therefore not necessary to use any Occupancy Permission System with the APB, though the Occupancy Permission System may be a handy tool for the dispatcher to control traffic. Some railroads use only informal dispatching in APB territory and this type of operation is listed as ABS or APB . Generic description of APB .
Trains can therefore occupy main tracks without further authority.
Within Yard Limits there is a much higher probability of another train or engine wishing to enter the Main Track in question. Even though these trains must comply with rules for entering ABS or CTC tracks, the risk of misunderstanding each others intentions, and misjudging who's moving under signal indication and who's moving at Restricted Speed is much higher than outside Yard Limits. Against the Current of Traffic Movement on Double Track within Yard Limits against the Current of Traffic is however not permitted, exept by Track Warrant, Track Bulletin or permission from Yardmaster or other authorized employee. Since CTC rules givern, no movement may enter a CTC track without permission from the Dispatcher. The permission is typically a signal indication but may also be verbal.