Jaipur railway's engine sound? (4)

I believe the answer is:

purr

I'm a little stuck... Click here to teach me more about this clue!

'engine sound?' is the definition.
(I've seen this in another clue)

'jaipur railway's' is the wordplay.
'PURR' is hidden within the letters.
But, I'm not sure how the hidden word is indicated.

Can you help me to learn more?

(Other definitions for purr that I've seen before include "Express contentment" , "Sound of happy animal" , "Sound of contentment" , "Sound of feline content" , "Noise of a contented feline" .)

I've seen this clue in The Sun.

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