There was a murder on the train, and only you can solve it! The key to solving this puzzle lies in collecting evidence and questioning the witnesses.
As soon as you enter this location, you will find yourself in the central part. If you crack this case open, your way to the birthday key is free!
There are four parts of the case you need to solve: motive, time, killer and food which as been used. Each category has four options, and only one of each is correct.
To mark the option you think is correct, just move the stones with the magnifying glass symbol onto the red target marks. However, remember – you can move these stones only after you have talked to all of the train crew and other passengers (all of them have orange exclamation marks above their heads):

Make sure to click on the quest giver (with the yellow arrow above his head) – he has all of the instructions. Also make sure to click and talk to each of the four accused people, they might have interesting facts to share with you.
Once you got familiar with the case, you can go to the next wagons on the left and right side. Each person has something to say about the case (they have two exclamation marks above their heads). Furthermore, there are some notes scattered in the rooms – check out all of them:

For example, one of the conductors claims that he saw a man early in the morning – could it be that the murderer was a male?

There is a dining wagon on the very left side, and the stewardess with the cook have also noticed some strange behavior:

After you have spoken with all of the crew members and read all of the hints and letters, it should be pretty clear who is the murderer, and what was the motive, time of the murder, and food used to get rid of our poor victim. Good luck!
If you are not a big fan of the detective movies, here is the correct setup of the hint stones (dať ako link): http://prntscr.com/nlhu71