Toy railway (5,3)
'toy railway' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for train set that I've seen before include "model railway" , "Strange treats in store for this model toy" , "Number of tracked items" , "A child might play with this" , "Layout with engines" .)
(Another definition for trainset that I've seen is " What every man is said to want".)