How do you get magneton to evolve in soul silver?

Answer 1

You can't. You need to trade it to Pokemon Diamond, Pearl, Platinum, Black or White.

The reason for this is that to evolve Magneton, you need to level it up at Mount Coronet. You cannot get to Mount Coronet in SoulSilver or HeartGold because it is in Sinnoh and SoulSilver and HeartGold are set in Johto and Kanto.

Although Pokemon Black and White are set in Unova, you can still evolve Mageton by leveling it up at Chargestone cave.

I would not advise evolving Magneton in the Black/White method because once you transfer it to Black or White you cannot transfer it back. Use the Diamond/Pearl/Platinum method if you want a Magnezone (the evolved form of Magneton) in SoulSilver or HeartGold and add it to your pokedex.

You could always trade for Magnezone. You encounter one in Jasmine's team when you fight the re-match.