Step on train carriage (10)
I believe the answer is:
stagecoach
'carriage' is the definition.
(I know that stagecoach is a type of carriage)
'step on train' is the wordplay.
'step' becomes 'stage'.
'on' says to put letters next to each other (in a down clue, letters appear on others).
'train' becomes 'coach' (synonyms).
'stage'+'coach'='STAGECOACH'
(Other definitions for stagecoach that I've seen before include "Large horse-drawn vehicle used to carry passengers and mail" , "Railway company" , "Great Western" , "Travelling" , "John Wayne film; passenger vehicle" .)