|This tower operator is distracted, but not unsafe|
|German Zs1 Signal|
The mechanism that allowed two trains to end up in a cornfield meet is the Zs1 signal, shown above. Known as an "Ersatzsignal" or Substitute signal it is placed below a "main signal" and lit upon command of the signaler when the main signal cannot be displayed normally. Many in English speaking rail circles have described it as a Restricting or call-on signal, but that analogy is not entirely accurate. In practice it is more like a manual block clear of the type that can still be encountered on the LIRR. Trains pass the Zs1 and proceed through all turnouts at no more than 25mph and before they can increase to normal speed . Some sources state the 25mph limit applies to the next main signal, but a majority say normal speed and that would agree with the behavior of the second train involved with the accident.
|Zs1 Displayed for a train movement|
|Bad Aibling Stellwerke|
In North America there is no signal that can be displayed into a CTC block where traffic is set in the opposite direction in the same way a Zs1 is used in Germany. You can't even give a Restricting against the flow of traffic, necessitating a permission past stop procedure and even then the train will continue to be limited to Restricted speed. The permission past stop procedure involves speaking to the locomotive engineer, usually over an open radio channel. Yes. dispatchers in North America can screw this up, but the procedure takes time, requires two persons and requires the operator or dispatcher to pay attention. In Germany the signaler presses a button and the train operator heads on his way. When used for routine problems like bulb out or axle miscounts the process can become rote and the procedure can be shortened or skipped entirely.
|The Zs7 Caution signal is the best analog to North American Restricting|
|313 absolute signal where the Zs1 was displayed before the collision. Annotated version.|
Second, the use of axle counters as a cheap alternative to track circuits, only serves to further ingrain the manual block thinking. Detecting track occupancy without track integrity only perpetuates the inability of distant signals to stop trains. Moreover, while axle counters are prone to failure (or miscount), just like track circuits, the signaler is able to reset the problem on his own with a button, instead of needing a maintainer to fix it. While some can see that as a feature, it removes a valuable form of two person control. Again, fixing signal problems via the interlocking panel becomes habitual and eventually an accident happens.
|Panel blocking was a basic way the PRR made operators think twice before bypassing signal protections.|
Note: This is my 261st post. Yay for important milestones!