Oberalppass, Switzerland, August 2001.
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This is the entrance of a rack section, system ABT, on the FO narrow gauge route between Oberalppass and Natschen.  To ease the insertion, the first teeth on the rack are rounded, lower, more spaced, and the beginning of the rack is mobile (a round spring, barely visible in the photo, keeps the insertion rack up).

Photo by Luca Logi <llogi@dada.it>