Why was one of the interurbans involved in the Bad Aibling collision not in a designated siding?
It’s believed the trains collided at or close to the track speed of about 62 mph. Neither engineer may have seen the other on the curved track until just before the moment of impact; both were killed, along with at least eight other people, and dozens were injured. “Black boxes” have been recovered and are being analyzed.

The eastbound train was to have been held at Bad Aibling, where there’s a passing track, until the westbound train arrived. Instead, the two trains hit about a mile east of town. Why did the eastbound engineer not know to wait for opposing traffic?

Moreover, both trains were fitted with PZB90 cab signaling, designed to sound an alarm and bring trains to a stop if there’s a danger of collision. How could the eastbound train proceed past a red signal without triggering automatic braking?
There will be a lot of speculation about mechanical failure.  Sixty years after Fail-Safe, can a capacitor failure still lead to a train wreck?

More troubling, though, is that "was to have been held" locution.  Recall, from our primer on positive train control, what the role of a timetable (which designates superiority by class and direction) is.
Note, in railroading, that a timetable does not REQUIRE a train to be at a station at the specified time.  Rather, it means that the train will not be BEYOND that station BEFORE that time.
Thus, if the Bad Aibling meet is stipulated in the timetable, and westbound trains are superior to eastward trains, the eastbound must wait.  No further signalling is required.  If the westbound is seriously delayed, the train dispatcher (I don't know the German equivalent job title) can change the meeting siding in a number of ways.  And if the dispatcher set up a meet on the fly, which is possible with centralized traffic control, the machinery will not permit conflicting routes to be set.  Thus one train must see a stop signal.

Curiouser and curiouser.

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