Learn English – The train commenced its journey – Is it bad word choice

There are some connotational differences. For example, a mental model of something which commences is a hollow cavity becoming filled up with activity, as opposed to something which starts as a ball which suddenly goes from rest into motion, or something which begins as a thing which slowly speeds up, or initiate, which is more like a pickaxe hitting a wooden dam, creating the first leak in what is presumably going to escalate into a quick flood. So the difference is that when something commences, things "inside" it undergo a state change; when it begins, it itself undergoes a gradual state-change, when it starts it undergoes a rapid state change, and when it initiates the state is changed into a rapidly-changing state.

To give a better idea of this, a court can commence criminal proceedings, which changes the state of the people in the court. Workers can commence construction. A country can commence preparing for war. These labels are all "containers" which hold people. But it would be weird to say "my bunny commenced eating her pellets" (started) or even "America commenced diplomatic contact with Iran" (initiated), since those adequately convey the start of the action but inadequately convey the state change.

For this reason, the train's journey commenced sounds awkward unless we are being invited to think about the people within the train and their status-change from "waiting" to "hooray, we're on a journey to a new place!". Most English speakers would probably say that the train began its journey. Furthermore the train commenced its journey sounds downright weird.

Since it is a less-common word, when you use it with an inappropriate connotation it often sounds unnecessarily formal.