Crossword Genius

Busier railway traversing mountains (6)

Ross

I believe the answer is:

sierra

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

'mountains' is the definition.
(I've seen this before)

'busier railway traversing' is the wordplay.
'SIERRA' is hidden in amongst the letters.
However, I am uncertain how this is indicated.

Can you help me to learn more?

(Other definitions for sierra that I've seen before include "Spanish word for a mountain" , "Spanish range of mountains" , "Jagged mountain range" , "A range of mountains in Spain" , "Mountain chain, S (radio comms.)" .)

I've seen this clue in The Telegraph.
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