Recollection of note on railway (6)
I believe the answer is:
memory
'recollection' is the definition.
(I've seen this before)
'note on railway' is the wordplay.
'note' becomes 'memo' (a memo is a short note).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'railway' becomes 'ry' (abbreviation for railway).
'memo'+'ry'='MEMORY'
'of' is the link.
(Other definitions for memory that I've seen before include "The ability to store things in mind, and to recall (6)" , "Something remembered" , "Recollection" , "Power of recall" , "commemoration" .)