For a train to travel to its destination over vast distances someone, usually an operator from a remote location, needs to keep the track switches on path. From what this movie depicts it is unlikely that the switch operator will stay intact long enough to make the train travel this far and still make it to its destination.
Incorrectly regarded as goofs
The abilities of the infected seem to change for the convenience of the plot. For example, before they claim the infected will only attack if they see you, Sang-hwa traps an infected man in the toilet and it still tries to attack him, despite not being able to see him. Also, it is clearly shown that the doors aren't automatic as the handle needs to be pulled down, but eventually they realise that the infected can't open doors, but during the initial outbreak several do so with ease.