Train set (6)
I believe the answer is:
school
I believe this is a double definition.
'train' is the first definition.
(I've seen this before)
'set' is the second definition.
(thesaurus)
(Other definitions for school that I've seen before include "Teach - fishes swimming" , "shoal of fish" , "a lot of swimmers" , "Group of sea creatures" , "group of swimmers" .)