DL has been operated the 757 between DTW & IND during the summer months (this year it's scheduled as a 321). I've been on a few of the DTW-IND flights (which depart after 10PM) through the years, and the highest load may have filled up a CRJ.
But January and February are "popular" months for lightly loaded aircraft. A couple years ago, I flew a few flights between DTW/SNA, operated with the 73G at the time, that had between 25-40 passengers on them (and yet the single flight, upgauged to the 757 shortly after, still managed to finish the year with a 90%+ LF). This year I've traveled on two of UA's "screw you Delta" $44 SNA-DTW (the fare was also available from MSP and ATL) flights. I've routed myself on the SFO-DTW flight each time -- on one flight in mid-January, there were so few people on the plane, everybody got a First Class meal.
Light winter loads aren't limited to air travel. We took Amtrak's Southwest Chief from Los Angeles to Chicago; there were two trains for coach (about 150 seats) and only about 30 passengers most of the way. Although the sun goes down early, it's a great time to sightsee and relax. When you get an entire row for youself, it's a lot more comfortable than a roomette.