Shoot a movie with a unifying theme

Those who know my on Facebook are very well aware of my relationship with railbound traffic.  Therefore, the local commutertrain in Nacka was a perfect choise for the first  film with a unifying theme.  

There is at least one goof in this short video. Can you spot it? 

I don't know if it count as cheating, but it tried to use some inspiration from next chapters in Digital Video Secrets. More about them in the coming blogposts. Stay tuned.