We don't need. It was good in this way no need more parallel world of Train
1: Actually then it's also a question how Do Won discover parallel dimensions? He was a witness when the train came into the station. Probably it was happened with Chief Oh, as well. Since she was a chief, she also visited the crimes scenes with the detectives, therefore she could been a witness of the train anytime too.
2: No, it means that you can't avoid your destiny, no matter what you do. Those scenes tried to show us his son always died. If he wasn't hang himself, then he cut his wrist, if he wasn't do these, he always went to a prison and something always happened with him. In short, no matter how much they tried to avoid their destinies, the result always have been the death. There was a time when Chief Oh put the handcuffs on his son's hand, but even if she did, he died somewhy, in different ways. Those scenes was about these. That was the point of them. She didn't get old, because it wasn't the same Chief Oh. It's like, try to imagine that there's endless dimensions (except our known one), and it's mean that you're living somewhere else, you have another self, with different life way, with different connections, but something is still the same: you can't run away from your destiny. No matter if you enter into another world, with another self. Chief Oh, Do Won, Seo Kyung, the detectives, everyone had unknown quantity selves. The one, we don't know is, which selves of Chief Oh was that, who killed his 2nd self. Probably the 3rd. Let's summary it.
A-world: Chief Oh-1 B-world: Chief-Oh-2 (she died by her another self), Chief-Oh-? (probably 3, who killed her own B-self and came from another world, probably the one, where Do Won and Seo Kyung were in the end, so C-world)
A-world: Seo Kyung-1 (died by one of Chief Oh) B-World: Seo Kyung-2 (he met with Do Won-A) C-world: Seo-Kyung-2 (she went in a 3rd world with Do Won-A)
A-world: Do-Won-A (he went to the B-world to save Seo Kyung-B after he lost Seo Kyung-A) B-world: Do-Won-A (met with Seo Kyung-B) C-world: (he went in a 3rd world with Seo Kyung-B)
3 : She probably went to the station where Do Won-A tried to kill himself, but then the train come and he probably get off. She knew which station he went, and it was raining that night. After she got to know about Do Won's location at the station, she ran there. She knew the train is coming in rainy nights after 21 o'clock. So she could have assumed that Do Won get off the train and everything gonna repeated later. But they were choose that to be together even if its a little and though everything gonna be repeated.
That's all girl, I hope it was understandable because english is not my native language and hopefully everything makes all sense.
https://www.soompi.com/article/1421913wpp/park-gyu-young-in-talks-along-with-got7s-jinyoung-and-ji-sung-for-new-drama
2: No, it means that you can't avoid your destiny, no matter what you do. Those scenes tried to show us his son always died. If he wasn't hang himself, then he cut his wrist, if he wasn't do these, he always went to a prison and something always happened with him. In short, no matter how much they tried to avoid their destinies, the result always have been the death. There was a time when Chief Oh put the handcuffs on his son's hand, but even if she did, he died somewhy, in different ways. Those scenes was about these. That was the point of them. She didn't get old, because it wasn't the same Chief Oh. It's like, try to imagine that there's endless dimensions (except our known one), and it's mean that you're living somewhere else, you have another self, with different life way, with different connections, but something is still the same: you can't run away from your destiny. No matter if you enter into another world, with another self. Chief Oh, Do Won, Seo Kyung, the detectives, everyone had unknown quantity selves. The one, we don't know is, which selves of Chief Oh was that, who killed his 2nd self. Probably the 3rd. Let's summary it.
A-world: Chief Oh-1
B-world: Chief-Oh-2 (she died by her another self), Chief-Oh-? (probably 3, who killed her own B-self and came from another world, probably the one, where Do Won and Seo Kyung were in the end, so C-world)
A-world: Seo Kyung-1 (died by one of Chief Oh)
B-World: Seo Kyung-2 (he met with Do Won-A)
C-world: Seo-Kyung-2 (she went in a 3rd world with Do Won-A)
A-world: Do-Won-A (he went to the B-world to save Seo Kyung-B after he lost Seo Kyung-A)
B-world: Do-Won-A (met with Seo Kyung-B)
C-world: (he went in a 3rd world with Seo Kyung-B)
3 : She probably went to the station where Do Won-A tried to kill himself, but then the train come and he probably get off. She knew which station he went, and it was raining that night. After she got to know about Do Won's location at the station, she ran there. She knew the train is coming in rainy nights after 21 o'clock. So she could have assumed that Do Won get off the train and everything gonna repeated later. But they were choose that to be together even if its a little and though everything gonna be repeated.
That's all girl, I hope it was understandable because english is not my native language and hopefully everything makes all sense.
Is it turned out? Maybe I just didn't notice...