A:
× When you transfer at Jyuso Station the Hankyu Takarazuka Line going toward Takarazuka Station is the train on the opposite platform form you so it’s an easy transfer.
✓ When you transfer at Juso Station, the Hankyu Takarazuka Line going toward Takarazuka Station is the train [across the platform]/[opposite the platform] from you, so it’s an easy transfer.
× Then, If there are many platforms at Jyuso Station, is it appropriate to use “opposite”.
✓ Then, If there are many platforms at Juso Station, is it appropriate to use “opposite”?
In spoken English, we're more likely to use "across".
Either way:
【形容詞のopposite】
"on the opposite platform" → 反対側のホーム(…遠い!)
【前置詞のopposite】
"opposite the platform" → ホームの反対側(…楽!)
It's also possible to say:
"on the *opposite side* of the platform"
...But that might give a false impression, so "across the platform" is probably best.
...That, or "on the other side of the platform".