Complex sentence: Seok-Woo realized that Su-an was not at her seat since she is trapped in a bathroom with Seong-kyeong, Jon-gil and the homeless man in a rear train with zombies.
Split into simpler sentences: Seok-Woo realized that Su-an was not at her seat. His daughter is trapped in a bathroom with Seong-kyeong, Jon-gil and the homeless man in a rear train with zombies.