Thanks for the info guys.
In working with HO cars, it was incredibly useful. No setting the car on the track, then bending over and squinting at t the first truck, trying to get the wheels on the track , and then the second truck, trying to get the wheels on the track.
I swear, that when the rerailer was in place, one swipe of the car forward through the section, and then back through the section, by hand, put it right on the rails. I don't think it was ever used for putting your engines on the tracks, just the cars.
I may try to get an HO rerailer, and see if I can fashion one for O gauge. It was a very simple device. I seen no reason why it wouldn't work just as well for the 3 rail O gauge, because the two wedges on each end would only force the two wheels onto the outer rails, not the smooth center roller.
These weren't used by pressing the car down hard on the track. You simply "floated" the car gently back and forth over the section while holding it in your hand, and suddenly the car was on track.