Railway car where middle button pushed back to load posh goods, article squeezed in (7,3)

'railway car' is the definition.
(I know that luggage van is a type of railway car)

'middle button pushed back to load posh goods article squeezed in' is the wordplay.
I cannot quite understand how this works, but
'posh' could be 'u' (abbreviation for upper-class - opposite to non-U) and 'u' is located in the answer.
'goods' could be 'gg' (an abbreviation for good written twice) and 'gg' is located in the answer.
'article' could be 'an' (indefinite article) and 'an' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

'where' is the link.

Can you help me to learn more?