Dic: a train of something, something in train (two related images)

■[N-COUNT] usu sing, N of n △A train of thought or a train of events is a connected sequence, in which each thought or event seems to occur naturally or logically as a result of the previous one.

• He lost his train of thought for a moment, then recovered it...
• Giles set in motion a train of events which would culminate in tragedy.

■[PHRASE] v-link PHR, PHR after v  △If a process or event is in train or has been set in train, it is happening or starting to happen. (mainly BRIT; in AM, usually use in motion)

• He praised the economic reforms set in train by the government.

... COBUILD