Visible crew/equipment: When Stepney puffs through the fog after dropping off the stone loads, studio equipment can be seen in the top left corner of the screen.
Revealing mistake: In a close up of Stepney at the beginning, his lamp iron has a burn mark.
Revealing mistake: When Stepney is returning home through the fog, the source to power his lamp is visible.
Answer: Breakdown trains help the engines back on track.