Throughout the cartoon, the shackles keep changing hands: from Sylvester's left hand and the bulldog's right, to Sylvester's right hand and the bulldog's left.
The length of the chain varies thru out the cartoon.
When Sylvester and the dog are hanging by the chain over the train tracks, Sylvester is hanging to the outside of the rails and the dog is hanging to the inside.
When they both peek up thru the rails to see the train bearing down on them, Sylvester is now poking his head up to the inside of the bridge and the dog is seen poking his head up to the outside.