I agree the centerline cars need some resistance to be really effective. I added quarters to the rollers and it helped. I also experimented with one car to add additional resistance which works great. Then I built two others of my own where I add a roller with one quarter inside of it to add resistance and they work really well also. You want the cars to scrub the track and not just roll over the dirt. What I like about the Centerline setup is that when it hits something like a point, it will roll forward over it easily. Here are some photos to show a little train I built up. You can see the difference in how much more the cars pick up with resistance added to them. I usually run the train dry but sometimes I've added a little alcohol to the fiirst roller. The first car in the last photo is a magnet car.
