Holiday on popular train (5,2)
I believe the answer is:
break in
'train' is the definition.
(thesaurus)
'holiday on popular' is the wordplay.
'holiday' becomes 'break' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'popular' becomes 'in'.
'break'+'in'='BREAK-IN'
(Other definitions for break in that I've seen before include "Illegally enter" , "Enter forcibly" , "Forced entry" , "Interrupt - burglary" , "Criminal entry" .)