The Lyric may be too poetic for popular Song. For those who know the Tay, and perhaps the bridge and train, it could easily engage them.
Ideally in Song the title, THE Hook, is mentioned a minimum of three times, to emphasize it as the main idea, the summary of the theme. You have two mentions, but the first one is buried as Line 4 in an 8-Line Verse I.
Melodically, the first use of the Refrain of that Line leaves me expecting more exposition. It doesn't seem to be summing up. The final giving of it does suggest the finality of the Storyline.
You get it done in 2:47 which is appealing.
'duffel coat' may need explaining, perhaps not to the local audience but to travel meaningfully the world, perhaps.
The words 'brittle vows' are obscure, poetic, but the word 'vows' is dropped in enunciation, which may rob the Line of meaning. To succeed in communicating obscure poetry enunciation is of strategic importance.

@gary-e-andrews Thanks for the comments, Gary. I always appreciate the way you take the trouble to take a more detailed look than most people are willing to do.

You hit the nail on the head with the "poetic" thing. This was originally a poem, but I find it hard to read a poem without a melody suggesting itself to me, so this happened 🙂 As a result, none of the normal rules are followed, particularly regarding the hook. The lyric has no chorus and no hook as such. I chose "Southbound Train" as the title because it conveys the metaphor and sounds cool. In some ways, "The Bridge" might have been better - a bridge to times past, to memories, to a dream...take your pick. My first idea for the title was "What You Did," which refers the listener to the unanswered question at the end.

In disregarding the "rules," I'm following much better writers than me. In fact, I'm not so much disregarding them as following a different pattern more suited to this particular type of song. Some of my favorite songs by my favorite artists do this. Here are some great examples:

Kate & Anna McGarrigle - Stella by Artois - no chorus, no hook, title not even mentioned

Dory Previn - Brando - no chorus, no hook, title not even mentioned. There's just too much to say in this song for a chorus to get in the way. It also does something I like and did in one song in using the title to tell you what the song is about 

Another one by the McGarrigles - Kiss and Say Goodbye - no chorus lyrically, the title mentioned once in the last line. Unlike the others, this is an upbeat pop song, but it still works. -

One of my favorite songs ever. I can't think of a more moving lyric and again a chorus or a hook would just get in the way. Unfortunately, it's in German, but maybe Bernd or Robert will find their way here. The title "Wir" simply means "we" and is the last line of the song. Reinhard Mey - Wir -

All of these folks know the rules and have great songs with wonderful hooks and choruses, but they also know when they get in the way and have other great songs that follow different rules more suited to what they are trying to do. I'm not comparing myself to them, just pointing out their inspiration to sometimes go down a different road.

Like you, I attach a lot of importance to clear enunciation. I went back and listened to the part you mentioned and it sounded clear to me. One thing I could do is have that first bass note come in a little later so that it doesn't overlap with the word "vow." Actually, I worry that that whole spoken part at the beginning is a bit cheesy, but nobody has said so yet.

I'm beginning to regret putting in that explanation about the Tay being a river as it seems to have people commenting as if the song were about the Tay LOL.

Yes, there is 'art' to be respected in Songs too, as well as broad commerciality. The 'Pop'/Popular Songs have to appeal to the lower common denominator, Lyrically, vocally, Melodically, in arrangement. But there's room, and even desirability in the eclecticism of 'art' Songs in the mix in one's own repertoire.
David Bowie's, "Cacti", on his Hunky Dory album comes to mind. Some kind of oddball poem, but it works in juxtaposition with all the other work on that album. I remember hearing it and not finding it out of place at all. 
If memory serves Dory Previn did theater work with Andre Previn. That Song seems to be like theater Songs, which are actually dialog for the actors, singing the ideas, the emotions, rather than simply speaking them.
Songs can be stronger, more memorable, with the 'rules', Structure, THE Hook clarifying the gist of a coherent theme. But there's room for breaking rules, bending rules, even writing new rules. As I say, having this one in the mix of your collective works gives an eclectic appeal I'm always looking for in people's work. Too many ABAB Rhyme-Schemes with obvious ideas tend to get boring. This one...I'll need to go read the Lyric again...but I think it's a simple fantasy for a girl seen in passing, and that's all it takes to make the rest more interesting in real life, speckled (dirty) windows, the train, the bridge, the river, the town's cobbled streets. All good visuals, and stronger once we have the Singer-Character and his object of desire to work with. A commute a man has made many times is enriched by a smiling face, a person moving in the landscape, a refreshing possibility, soft flesh imaginings, juxtaposed with brittle vows.
There's no 'wrong' way if a Song's way of telling it serves a purpose to the writer. Not every Song has to be for everyman's approval. There's a virtue in the benefit to the writer, and those to whom it does appeal, does stimulate similar emotions or ideas. One should never presume others' goals are the same as one's own. And, unless we're been reading their diary, we can nought but presume. Behold! I wax poetic. But, let's face it. Poetic needs a good waxing. 

You make a good point about varying things a bit, as in the Bowie example. Another great example of not following the rules is Simon & Garfunkel. "For Emily, Wherever I May Find Her," "Kathy's Song," "Bookends" - all classics that found their way onto the Greatest Hits Album with no chorus, no hook and no occurrence of the title in the song.

Dory Previn was married to Andre Previn until he left her for Mia Farrow. They collaborated on a lot of music for films, but her later work which she performed herself was very introspective, tackling some quite dark themes but always very melodic with some very catchy tunes thrown into the mix. She was also a pioneer of music video before it was a thing. The BBC had a show called "The Camera And The Song," which was a filmmaker creating a film around a song. It was an unusual concept back then in the 70s and she was one of the artists featured. I would absolutely recommend checking out her music. She's pretty unique.