Visible crew/equipment: When Bill and Ben are shunting, we can see hands. (00:02:00 - 00:10:00)
Visible crew/equipment: In the restored version, when Bill and Ben leave Brendam Docks, the twins do not disappear. They just stop right at the top of the screen. The end of the set can be seen in a still from the episode: also, two hands can be seen pulling them backwards with string.
Visible crew/equipment: When Bill and Ben leave Boco and Edward, a tripod is visible in the top right (visible in HD full screen version).
Visible crew/equipment: When Bill and Ben pass a station, you can see a bit of studio equipment at the top of the screen. (Visible in HD full screen version).
Visible crew/equipment: When Bill and Ben go by a level crossing, the top of the set is visible.
Answer: He was in the water.