Just to add a couple of comments on ore cars as used on the GN:
Not only did the ore cars spend half their active life running empty,
they spent half the year sitting idle, as the shipping (no pun
intended) season for ore ran from approximately April to Oct/Nov during
much of the steam era.
On the GN, the ore "jennies", as they were called, were also pressed
into service for the sugar beet rush during the late fall/early winter
timeframe. Of course, they were inefficient, but when GN was short of
GS gons or hoppers, they were used. I haven't been able to find photos
from the steam era, but I have personally witnessed this on BN (ex-GN)
trackage around Grand Forks, ND in the early eighties timeframe.
Robert D. Heninger