Busier railway traversing mountains (6)
I believe the answer is:
sierra
'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.)" .)