My winter route follows mostly dirt. In the morning, it is usually frozen. With the mild days we've been having, it has been thawed for the evening run.
The tracks on the rail trail indicate that a lot of people (for this time of year) have been waiting until the mild part of the day to take their rides. Why do people choose to slog through inches of glop just so they can wear a bit less clothing? I could understand if the ride was on pavement, but this is all dirt.
You do get quite a workout, even when the soft layer isn't deep. The stickiness and suction of the wet silt drag you down at least a couple of gears. The bike sucks to a halt unless you keep constant power to the pedals.
Some mornings have followed mild, wet nights. My route is basically a 7.5-mile descent in the morning. Some of it is steep enough to push up to 30 miles per hour on firm track. The freeze-thaw cycle might leave frost a couple of inches down, with the sticky layer on top. A longer thaw softens the ground more completely. Either way, you don't have a fast, firm track. And the grind back up at night is as much work as you might imagine.
What strikes me on a frozen morning is how many people went out when it was soft, and how most of them took suicidally shallow lines through the rail crossings that plague the Cotton Valley Trail. The ruts they leave present a special hazard to the hurrying commuter pushing the pace on the downhill run when they freeze overnight. They'll suck a tire in before you can pull out, leading you into the same flat line that the rider took to make them.
Riding requires observation and analysis. That's part of what makes it fun. It's a bummer that frozen ruts might disrupt the fastest line through a particular trail feature, but that's one aspect of public trails. The challenge is to find the best line through conditions as you encounter them.