Running Three-Rail Model Railroad Trains

With the throttle again set back to zero, and your locomotive situated ahead of an open straight area on your track, rail and connect your cars behind the locomotive. Roll each car back and forth a few inches like you did with the locomotive, to be certain they are properly railed. When you are satisfied that a car is properly railed, couple it to the car adjacent to it by bumping them together gently. Some O scale couplers don't have a hinge, like the one on the right in the photo. These you'll have to lift the end of the car in order to couple them to the adjacent car.
When all your cars have been properly railed and coupled, slowly back your locomotive up to the string of cars until the coupler connects. You may have to push the lead car against the locomotive gently to get them to couple or even lift the end of the car if the coupler doesn't hinge.