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

[Cohen, Gerald Leonard]

   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."

    I don't have OED or DARE handy, and so maybe "cat hop" appears there.
But in any case I don't find it in HDAS or Casell's Dictionary of Slang.
Would anyone in ads-l be familiar with this term and how it came to mean "right on time"?

Gerald Cohen