I've driven past this very spot over 4,500 times (more than double that if you count going the other way at the other part of the day)...and I have noticed this effect, but only yesterday it sunk in.
As I come into town, I can see the train lines as they come into/out of town. One line is oiled heavily, the other is pristine...
Train lines being what they are, the traffic in both directions must be the same, bar a possible few handfuls of retired trains.
Trains on the left are climbing s slight grade into town (grade saves brakes as they slow into town)...they are also loaded (typically with coal) as they head that way towards the coast (power stations and export)...all engines running.
Thus the ones on the left are running hard, at load, and blowing by their crank breathers...on the right, they are on the return trip, unloaded, and still powering slightly, with not all engines running.
This is pretty typical, three engines (first two were running), and empty coal cars (100 tonne each, 80 tonne coal to fill them)...
Amazing what a difference 3,500 tonnes on the drawbar will do.