A standard prio gives priority to trains on the mainline and makes trains on the sideline way. In some join, especially at BBHs, a prio may build up long queues waiting to enter the mainline. On PSG 178, we experimented with a concept named a Two-way Prio. The basic idea is that when the waiting trains begin to roll, they should keep rolling and not stop for a few trains on the ML. The mechanism is similar to a real-life traffic light. It releases all traffic in one direction and then switches to the other direction and releases all traffic from there (although, the Two-way prio does not have a timeout, like traffic lights).

A place to use this is in BBHs where neither of the MLs is more major than the other. A simple join will leave huge gaps, and a standard prio may build big queues even with good balancing.