We are never shown how Hae Suk looked like in the past. .....
My guess is somehow Somi is Hae Suk's forgotten past. I don't know how she exists. But that's the only half logical thing I can think of.
Remember she said that she lived with her mother in law for 5 years. Eun Ho is also 5 years so there might be some connection.
Somi might be destined to hell because she abandoned Eun Ho or may be she tried to kill her self and Eun Ho by drowning after having unbearable treatment from her mother in law? Idk just a guess. Anyhow because of some accident or some brain damage she doesn't remember it.
Somi got on the train almost immediately after Hae Suk, so may be this memory lost part of Hae Suk's soul was wondering around on earth and had to got on the train after Hae Suk died.
Nak Jun instinctively saved Somi from going to hell because she looked like Hae Suk's younger self. But hid the facts from Hae Suk since he didn't want to hurt Hae Suk with bad memories.
Spoiler for light in your eyes. The writer is known for these brain wrecking writing. She did that in Light in your eyes as well. Even Han Ji Min played the younger version of Kim Hye Ja there as well. We didn't even know all those things that happened in the drama was happening inside the demented ladies head until the very end.
1 
But cinematic vice, indeed a masterpiece.
Also acting is pretty good too.
I'm not much of a fan of the script though
I really liked her glare at HongRang
Those eyes were burning with rage
This OST ❤️❤️❤️
That was her 4th Daesang award.
Remember she said that she lived with her mother in law for 5 years. Eun Ho is also 5 years so there might be some connection.
Somi might be destined to hell because she abandoned Eun Ho or may be she tried to kill her self and Eun Ho by drowning after having unbearable treatment from her mother in law? Idk just a guess. Anyhow because of some accident or some brain damage she doesn't remember it.
Somi got on the train almost immediately after Hae Suk, so may be this memory lost part of Hae Suk's soul was wondering around on earth and had to got on the train after Hae Suk died.
Nak Jun instinctively saved Somi from going to hell because she looked like Hae Suk's younger self. But hid the facts from Hae Suk since he didn't want to hurt Hae Suk with bad memories.
Spoiler for light in your eyes.
The writer is known for these brain wrecking writing. She did that in Light in your eyes as well. Even Han Ji Min played the younger version of Kim Hye Ja there as well. We didn't even know all those things that happened in the drama was happening inside the demented ladies head until the very end.
Check this channel out for OST BTS
https://youtube.com/playlist?list=PLr0T5CaHaPwXdVcxTFZL_zJCYZxlXuj2Z&feature=shared
This one for interviews, BTS, possible updates on S2s etc