So, there's a number of ideas associated with this.
Firstly is the story element. In the Victorian era, passengers boarded a train, but when the train arrived at the station all of the humans had dissapeared and the interior of the train shows signs of extreme distress despite no physical change to the outside. I don't know the exact conditions yet, but the cause of this is a dimensional anomoly that basically kidnaps all of the passengers over the duration of the journey. Nightmare creatures probably manifest on the train and spread a sort of infection so the interior of the train is covered in weird stuff then attack the human passengers before dissapearing back to their dimension.
The SCP itself I think is the train, which is still functional. Passengers on the train are made to fall asleep somehow and then relive the memories of the passenger who was in their seat from the time when the anomoly occured. Some passengers will be be informative than others. The test logs will start with an engineer or driver who sees very little and is killed from behind, then maybe a conductor who enters a passenger compartment while the anomoly is in progress and is killed quickly but sees little, then a passenger who sees the whole thing happen from beginning to end.
There's also the tertiary issue of the defined nature of the event, as I know extradimensional or demonic type things have been done time and time again. Not sure what to do about it yet.
Edit: developed it a bit in my sandbox.