It can be very frustrating having a locomotive that you need to push to get going, or it suddenly speeds up and falls off the tracks.
A locomotive runs by picking up the electricity from the track through its wheels. The wheels transfer the electricity to the motor, which then turns the gears to drive the locomotive.
A locomotive with poor pickup on the wheels or a poor gear set up will give you lots of problems. As with most things, you get what you pay for... but this is one area you do not want to skimp on. A great operating locomotive is 90% of the way to having a fantastic model train layout.