I read this discussion with quite a bit of interest. I'm a Web II parent, so getting ready for my 5th derby as a "derby dad" in a few weeks. I also have a Bear so I'm good for a few more years after this. I've been hosting workshops in my garage for the past four years, and I will spend as much time with the kids as their interest demands. Or I try to, the workshops have become more popular over the past two years. I do the same for my boys, as long as they are interested I will help. When their interest wanes, work on their cars stops.
This year my oldest finally realized that my entries in the "outlaw" (parents and siblings) races were always faster than his cars were, and he wanted to know why. I never pointed it out, he just this year finally realized that my times were faster.
I asked him, "How many steps do you take to polish your axles?" He replied, "Three." I said, "I do eight. If you want to spend the time to do all of those, I will show you what you need to do." This year he did all of the steps (mostly just additional graduations of wet sandpaper.) Not to be outdone, his younger brother did exactly the same thing!
I had one dad ask me during the workshop what to do when his boy wasn't helping with his car. I told him, "Stop working on it."
Last year my older boy did a science project for school where we drilled 3/8 holes on top of an otherwise unweighted block car. We ran 5 races unweighted. We ran 5 races with 2.5 ounces of tungsten cylinders in the front of the car. Finally we ran 5 races with the 2.5 ounces of tungsten in the back of the car. We posted his science project in the garage this year during our workshops, so anyone who asked about weight placement got to see empirical evidence as to what the answer was.
So in a nutshell, I share whatever tips I can, but they're based on what level of interest the boy has. I've never personally made a rail rider so I don't have any expertise in that area. I think I'm decent at helping the boys coax their creative vision (car designs) out of the raw block of wood, but I suck at alignment and I tell people that. I've probably made rail riders by accident.
But anything I know, I'm happy to share.