Instructions
Go to the Power Plant at the end of Route Nine, east of Cerulean City. Talk to the plant manager inside and he will tell you that someone has stolen a machine part vital to the running of the plant.
Head back west to Cerulean City and go into the Pokemon Gym. A member of Team Rocket will run out and escape to the north. Follow him north to the bridge on Route 24 and he'll admit hiding the machine part somewhere in the gym.
Go back to the Cerulean Pokemon Gym and find the machine part at the top left of the swimming pool, underneath some rubber rings. Take it back to the Power Plant to restore power to the Kanto region and the Magnet Train.
Go to Vermillion City through the Underground Path and go to the Pokemon Fan Club. Talk to the man inside and he will give you a lost Pokedoll.
Go to Saffron City and go into the house in the north of town next to the Silph Co. building. Go upstairs and give the lost Pokedoll to Copycat and she will give you the Magnet Pass.