Slogan displayed by Pullman Trains (6)
I believe the answer is:
mantra
'slogan' is the definition.
(I know that mantra is a type of slogan)
'displayed by pullman trains' is the wordplay.
'displayed by' indicates a hidden word.
'MANTRA' is hidden within 'pullman trains'.
(Other definitions for mantra that I've seen before include "Buddhist chant" , "word to help concentration" , "Word or phrase repeated as aid to concentration" , "Something said to focus concentration" , "repeating phrase" .)