Overnight berth that's underlying track (7,7)

'overnight berth that's underlying track' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'overnight berth that's underlying track' is the wordplay.
'overnight berth' becomes 'sleeper' (I can't justify this - if you can you should give a lot more credence to this answer).
'that's underlying' says to put letters next to each other (in a down clue, some letters lie under others).
'track' becomes 'railway' (railway is a kind of track).
'sleeper' put after 'railway' is 'RAILWAY SLEEPER'.

Can you help me to learn more?