Exasperated by a useless railway! (5)

I believe the answer is:

angry

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

'exasperated' is the definition.
(thesaurus)

'by a useless railway' is the wordplay.
'by' means one lot of letters go next to another.
'useless' becomes 'ng' (I can't justify this - if you can you should believe this answer much more).
'railway' becomes 'ry' (abbreviation for railway).
'a' put next to 'ng' is 'ang'.
'ang'+'ry'='ANGRY'

Can you help me to learn more?

(Other definitions for angry that I've seen before include "Inflamed" , "Vexed" , "Very annoyed" , "Irate, mad" , "Incensed" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.