Seaside railway? (9)

I believe the answer is:

coastline

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'seaside railway?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

'seaside railway?' is the wordplay.
'seaside' becomes 'coast' (I've seen this before).
'railway?' becomes 'line' (line is a kind of railway).
'coast'+'line'='COASTLINE'

Can you help me to learn more?

(Other definitions for coastline that I've seen before include "Outline of land from sea" , "Where the sea meets the shore" , "Shore boundary" , "Seaboard" , "Shape of the seashore" .)

I've seen this clue in The Sun.

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