Rick,
that you called it a passing siding, that's exactly what we called 'em here, I didn't think I should use that term on here though. Very common in useage at Idaho, no less than 7 in I.S. alone, must make for the most complex industrial switching area in all of the Colorado N.G. which in further I.S.Mill installments I shall relate.
Here's a 1920's view of the same siding "still in"...Even after the Bertha was demolished, the passing siding remained, I'm not certain if the stub siding did though. Must have made a good place to stash loads-out for easy pick-up on the return to Denver.
http://digital.denverlibrary.org/cdm/singleitem/collection/p15330coll22/id/3369
UpSideDownC
in New Zealand