railroad slang: "on the cat hop" (right on time)

[Douglas G. Wilson]

>    I've been asked by a history professor  about an expression he came
> across in his research on the railroads: "on the cat hop" (right on
> time), e.g., "The freight train left on the cat hop."

"Cat hop" has been used in various ways, but I think the basic sense here
would be "pounce". The train "pounces" away from the platform after waiting
catlike for its proper time. This analogy might have been particularly
appropriate to one who had arrived seconds late and missed the train. Just
a speculation. This hypothesis would assume that the original context was
like "departed/started on the cat-hop", with any other contexts (such as
"rolling along the track, on the cat-hop" or "pulled into the station on
the cat-hop") appearing only after the expression was equated to "on time".

-- Doug Wilson