。
It's smart to go by train.
Literal
Train [by-で] go [nominalizer-の] [subject-が] smart [copula-だよ].
A ~のが+adjective evaluation. The nominalized verb phrase 列車で行く ('going by train') becomes the subject of 賢明 ('wise, smart, sensible'). 賢明 is a な-adjective here in predicate position + the informative よ sentence-final.