You can get the Magnet Train Pass from the Copycat Girl NPC in Saffron City. She will give you the pass after you recover her lost Clefairy PokéDoll from the Pokémon Fan Club in Vermilion City.
But the current owner of the doll will only return it after you’ve talked to the Copycat girl at least once to start the side-quest.
The Pass is necessary to use the Magnet Train, which connects Goldenrod City in Johto to Saffron City in Kanto. It’s the fastest and easiest way to travel between both regions.
Just note that you’ll have to fix the Power Plant near Cerulean City first before you can ride on the Magnet Train.
You’ll need to be in the post-game and have access to Kanto before you can get the Magnet Train back online and acquire the Pass.
But once you make it to Kanto the process is fairly simple:
Just go to Saffron City and start the side-quest of finding the Copycat Girl’s missing doll. Once you complete the quest you’ll get your own Magnet Train Pass.
Getting the Magnet Train Pass (Step-by-Step)
Step 1: Finding the Copycat House
You’ll find the Copycat’s house in Saffron City, just two streets above the Pokémon Center.
Go inside and head up to the second floor to look for the Copycat. You’ll find her sitting at a table in her room.
She’ll take on your character’s appearance when talking to you, which can be confusing – but all she’s saying is that she wants you to find her Clefairy PokéDoll in exchange for a Pass.
And to find her doll we’ll need to head to Vermilion City.
Step 2: Finding the Copycat Girl’s Doll
Fly to Vermilion City and then head to the Pokémon Fan Club.
This is the building directly south across the water from the Pokémon Center.
Talk to the Pokémon Fan on the left side of the table to retrieve the doll (you’ll see this NPC sitting next to the Clefairy PokéDoll at the table).
He’ll gladly return it after hearing about the Copycat Girl and how she misses her doll.
Step 3: Exchanging the Doll for the Train Pass
Return the PokéDoll to the Copycat Girl in Saffron City and you’ll receive the Magnet Train Pass as thanks.
How To Fix the Magnet Train
Getting the Pass isn’t enough to board the Magnet Train right away. You’ll also have to get the Magnet Train back online.
To fix the Magnet Train, you must restore the Kanto energy grid by retrieving the missing Machine Part and bringing it back to the Kanto Power Plant. This is a separate side-quest that takes place in & around Cerulean City.
Once you’ve returned the Machine Part to the Power Plant manager, the Magnet Train will have electricity & it’ll be open for business once again.
Note: Using the Magnet Train is 100% free and you can ride it an unlimited number of times.