Head East out of Cerulean City fighting trainers and wild Pokémon until you reach the end of Route 9 where you can surf to the Power Plant. Go inside the Power Plant and talk to all of the people in there. The fat man is really upset because someone stole a Machine Part
Once you try to exit, the Guard will say that some shady character in Cerulean has been spotted and asks for your help. Now go back to Cerulean City.

Once in Cerulean, go inside Misty's Gym. The Rocket member who stole the Machine Part will bump into you and exit as fast as he can. Go to the top left of the pool in the middle of the gym path and press A as if you awere about to surf and you'll find the missing Machine Part.
Now go back to the Power Plant and give the guy the missing Machine Part. He'll thank you and give you TM 07, Zap Cannon.
Now head back to Cerulean City again, but why go there when you can get the Magnet Train Pass

Now that the Power Plant is giving power to the Magnet Train, you can get a ticket for the Train which will transfer you between the two worlds a lot faster than any other way.
First, head back to Saffron City and talk with the Mimic Girl. She'll tell you she lost one of her Poké Dolls.
Then head to Vermilion City to the house where you get the bike voucher in the previous versions. Talk to the man above the Doll inside. He'll give you the Poké Doll that you'll give to the Copycat Girl back in Saffron.
Now go back to Saffron, and give her the doll. She'll give you a Pass to the Magnet Train that will let you ride on it anytime you want.
The Train terminals are in Goldenrod City next to the Radio Tower, and in Saffron City next to the gyms..

Now head back to Cerulean City

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