Drama started off strong but soon became draggy and over dramatic. There are a lot of parts added just for the dramatic effect, that did not make much sense.
(Spoiler warning - Don't read on if you have not finished the drama!)
The ending where the MC sacrificed himself was especially silly. I could think of other ways where he could have save himself and still gotten out of the situation. Surely he would have thought of it since he was resourceful and intelligent. For example, he could have demanded the soldiers to move into the other car while he stayed on the same car with the hostage, and still shot the car connector off so that the soldiers were left behind. The soldiers didn't know he only had one bullet, and thus, wouldn't have shot back risking the VIP hostage. Or he could have demanded the soldiers to jump off the train.
Also, during most of the show when Lee Wan-Ik was being a menace and a threat. I kept thinking why didn't the righteous army just kill him off since it was not that uncommon for ministers to be killed. In the end, he was still assassinated but it could have been done much earlier, instead of waiting him to cause as much damage as he did. Obviously, they didn't kill him off sooner because they needed to create drama, but it makes for a weak story.
There were a lot of other plot holes, which is unfortunate because the acting throughout was pretty good.