The train commenced its journey - Is it bad word choice?

Solution 1:

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.

Solution 2:

There're synonyms for verbs, indeed. The choice depends largely on the usage (formal vs informal or written vs verbal) and context.

Examples:

The New Horizons probe commenced its historic journey on January 19, 2006.
Police investigation began soon afterwards.
Movie begins at noon.
Engine wouldn't start.
I started the engine.

commence - formal, usually in written language
begin / start - informal, more common and often can substitute each other

While choosing a synonym may not be wrong, it might sound different (or pedantic) to the readers or listeners. If you're a learner, it's recommended that you practice using the language like its native speakers do.

Solution 3:

A word can express many implications, connotations, and attitudes in addition to its basic “dictionary” meaning. A word's near-synonyms differ from it solely in these nuances of meaning.

The traditional usage supports the choice of commence in reference to: court proceedings,religious or other ceremonies, or industrial, commercial or military ops.

• The government will evaluate the land grab practices that commenced in 2000. (Time,Jul 15, 2015)
• Lawsuits commenced, and the partnership eventually settled..(New York Times,Jul 2, 2015)
• Here we go Let the agony/ecstasy/apathy/flag-waving commence. (The Guardian,Jun 9, 2015)

Distinction: (M-W dictionary of synonyms, pg 94)

Start implies opposition to stop. It therefore suggests a setting out from a particular point (as on a journey or course) often after inaction or waiting.

• The train started....
• The conversation started, then stopped and after a pause started again.
• The horses are ready to start...