The thing is, train consists aren't random. They follow the timetable just as much as anything else does. Peak hour trains tend to be longer and heavier then a off-peak or even night time train. Having a double-unit train off-peak would be unrealistic, as it's a waste of resources, and a peak hour train with only one unit would not be able to cope with the demand.
And as a driver, you don't get to choose the loco you're driving. You're assigned one by the loco disposition.
Freight trains tend to have more fluctuation, but they follow the demand and supply rule as well, only they aren't tied to peak or off-peak times.
What I could see would give some alternation when driving freight trains, would be randomised train lengths, maybe even have a low chance off the loco running light when it actually should have a train to pull. This way, not every train is the same each day/hour.