Railway employee switching to steam trains (13)
I believe the answer is:
stationmaster
'railway employee' is the definition.
(I've seen this before)
'switching to steam trains' is the wordplay.
'switching' indicates an anagram (letters switch places).
'to'+'steam'+'trains'='tosteamtrains'
'tosteamtrains' is an anagram of 'STATIONMASTER'.
(Other definitions for stationmaster that I've seen before include "Railway employee" , "Railway official" .)