I usually find that a good rule for creating thesis statements about poems is to fill in this sentence: In ["Poem's Title,"] [Author's Name] uses [literary device], [literary device], and [literary device] in order to convey the idea that [central purpose]. A well-developed thesis should include not only the poem's main message (or central purpose) but also the ways in which the poet produces and conveys that purpose (the literary devices employed to achieve it).
A good thesis might look something like this: In "The Convergence of the Twain," Thomas Hardy employs metaphor, form, and a particularly descriptive title to convey the idea that the sinking of the Titanic was the inevitable result of humanity's pride.
The facts that it was "human vanity" that conceived of the Titanic and our "Pride" that planned the ship are referenced in the first stanza (lines 2-3). Further, the comparison of the iceberg to a "mate" for or a partner of the Titanic helps to convey the idea that they are meant to come together (19). The phrases "intimate welding" (27) and "twin halves" (30), as well as the word "consummation" (33) also emphasize the idea that the ship and iceberg amounted to two parts of one whole. The idea of mates, of intimacy, and of consummation (a physical union, but also an ultimate end) all lead to the idea of the two seemingly "Alien," or unrelated, things actually being designed for one another (25).
Further, the title uses the word "Convergence" which is not typically how we would think of the coming together of the Titanic and the iceberg. We might call it a crash or a sinking or something that employs a more violent or tragic connotation, but "Convergence" actually just means moving to join together -- again, the idea of the two things being destined to combine is clear. Moreover, the word "Twain" means a pair or couple, and so this matches the meanings of "mates" and "twin halves" too.
Finally, each stanza of the poem employs end rhyme, where the last word of each of the three lines rhymes with the others. This rhyme scheme makes it seem as though the three lines "belong" together. Plus, the first and second lines of each stanza are significantly shorter than the third line, and this actually makes each stanza look like the shape of a ship or an iceberg above the water. More importantly, though, is that the first line's length combined with the second line's length would equal the third line's length. The first and second lines, when combined in length, come together to approximately equal the third line's length. This also enhances our reading of the poem by repeating, again and again, a coming together of two things to equal something larger: just as the Titanic and iceberg converged to create "one august event" (30).