You're welcome. Same here in the U.S. as far as wheels go. Practically all freight cars are equipped with modern, roller bearing trucks and therefore have the rusty-looking wheel faces. In fact, it's a requirement for cars in interchange service to have RB trucks...so lots of rusty wheels!
It's the older cars, like SOME of the ones you have weathered, that would have had the friction bearing trucks, and the greasy black/gray wheel faces.
FWIW, I mix up my own custom blend of acrylic paints to represent this color. I start with black semi-gloss (to give it a little bit of "greasy shine"), and then add some Grimy Black (which looks like a dark gray) and just a touch of dark brown.
Then, when I'm actually painting the wheel faces, I dip the brush in black weathering powder and mix it in with the paint. This step adds texture, giving the appearance of caked on dirt/grime in the "grease".
Keep up the great work!