GM:
The Chicago Terminal normally sticks to the same Tuesday/Thursday schedule that CP Rail maintained. However, shortlines are typically more agile and responsive than larger railroads and can offer on-call or as-needed service on other days.
Finkl may have requested this special movement that you saw. To be fair to CP Rail, I can recall seeing CP Rail/Soo Line operate on a few Saturdays over the last two decades too, however though it was very rare.
Tom