On most railroads with timetabled passenger trains using the same track in both directions, there is a clear protocol by which one train waits at the end of a side track to clear the movement of another train.

Here is the protocol being observed on the Wellington to Johnsonville section of New Zealand's railroads.  This branch looks like William Jackson Palmer hired John I. Beggs to build the tracks.

Khandallah Road Station, 6 January 2000.

Note that the signal at the far end of the platform is red.  This is a STOP AND STAY aspect, and it will clear to permit the train I'm on to descend the steep grade into Wellington station.  The signal at the opposite side of the approaching train is also STOP AND STAY.  Modern signal installations generally set all signals governing opposing movements either to STOP OR STAY, if at the end of multiple track, or to STOP, in case a train runs past the STOP AND STAY and enters the single track.  That (improper) entrance will also set signals facing the approaching train to STOP.  An investigation is embarrassing to the carrier, and some crew members will be disciplined, but that's better than broken trains and dead people.

Here we see the same protocol at work on the South Shore Line.

Sheridan siding, Michigan City, Indiana, 13 August 1966.

The approaching train is leaving the single track that runs through the middle of Michigan City streets.  There are two levels of protection at work here, the red signal and a line in the employee timetable establishing a meet here.  Recent South Shore schedules have moved many of the scheduled meets elsewhere, because of heavy passenger loadings affecting the timekeeping, particularly of rush-hour trains.

The latest news out of Germany suggests that in the Bad Aibling collision, the dispatcher and the train crews neglected to take the safe course -- with one of the trains running four minutes late?
Investigators currently believe that the controller mistakenly allowed both trains to enter the single stretch of track at the same time.

While a warning signal is supposed to be set off in such circumstances, it is possible to circumvent it.

If this is how events took place it would seem the controller only noticed his mistake at the last second - as indicated by the use of the emergency call.
There's more in the Süddeutsche Zeitung, auf Deutsch.

History rhymes.  In the fatal Speedrail collision in 1950, the southbound train accepted a permissive signal, and the motorman of the northbound train, also president of the railroad, ordered an employee to line his train for the main line, against a signal that turned out afterwards to be a stop signal.  At Bad Aibling, it appears that the dispatcher allowed a train to pass a stop signal, only to attempt to reverse his error later.  In both collisions, curves reduced the line of sight for the operators.

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