A Simple Twist of Fate

Joyce quotes are italicized 
Dylan quotes are bolded

So far we have seen Stephen's street ramblings change from a light, willful participation in a fantasy world (Blackrock) to a seemingly addictive, distorted, alien world (Dublin). The change can be "diagnosed" in part by more "tangible" causes--the lower social class, the increased sexual frustration, religious anxiety, etc. There may also be a more abstract element of "coming of age" that cannot be explained by events that effects the drastic difference in tone we see between the different ramblings. 

The Blackrock fantasy was much more grounded in the Count of Monte Cristo. It was much less confusing, very sunny, etc. 

There would come to his mind the bright picture of Marseille, of sunny trellises, and of Mercedes

The difference between the two is best shown in this passage: He returned to his wanderings. The veiled autumnal evenings led him from street to street as they had led him years before along the quiet avenues of Blackrock. But no vision of trim front gardens or of kindly lights in the windows poured a tender influence upon him now.

The appeal of a Mercedes comes from Stephens desire for an intimate relationship, and his inability to go about getting one (the tram scene is an example). He describes his Mercedes fantasy in the following passage...

He did not know where to seek it or how, but a premonition which led him on told him that this image would, without any overt act of his, encounter him. They would meet quietly as if they had known each other and had made their tryst, perhaps at one of the gates or in some more secret place. They would be alone, surrounded by darkness and silence: and in that moment of supreme tenderness he would be transfigured. He would fade into something impalpable under her eyes and then in a moment he would be transfigured. Weakness and timidity and inexperience would fall from him in that magic moment.

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I think that the Bob Dylan song "Simple Twist of Fate", both musically and lyrically fits Stephen's street ramblings. 

I'll start with some Dylan lines. 

They walked along by the old canal
A little confused, I remember well
And stopped into a strange hotel with a neon burnin’ bright
He felt the heat of the night hit him like a freight train
Moving with a simple twist of fate

A saxophone someplace far off played
As she was walkin’ by the arcade
As the light bust through a beat-up shade where he was wakin’ up,

The imagery feels very similar. It's used to create a sense of loneliness surrounded by a bustling world. The protagonist in Dylan's song finds himself alone in the middle of the city of neon signs (neon always implies nightlife) and saxophones. There is this very real and very alive city that is completely separated from the protagonist. 

He hears the ticking of the clocks
And walks along with a parrot that talks
Hunts her down by the waterfront docks where the sailors all come in
Maybe she’ll pick him out again, how long must he wait
Once more for a simple twist of fate

Again, we have the lone soul walking among the bustling city docks. This time, he's hoping "she'll pick him out again" as he waits for a simple twist of fate.

In Portrait, we get very similar passages.

He passed unchallenged among the docks and along the quays wondering at the multitude of corks that lay bobbing on the surface of the water in a thick yellow scum, at the crowds of quay porters and the rumbling carts and the ill-dressed bearded policeman.

He had wandered into a maze of narrow and dirty streets. From the foul laneways he heard bursts of hoarse riot and wrangling and the drawling of drunken singers

By day and by night he moved among distorted images of the outer world. A figure that had seemed to him by day demure and innocent came towards him by night through the winding darkness of sleep, her face transfigured by a lecherous cunning, her eyes bright with brutish joy. 

These are all images of someone who doesn't know what else to do than look for a simple twist of fate--his Mercedes to step out of the shadow... or to "pick him out again". 

There are a lot more more parallels you could draw. The first passage in "Simple Twist of Fate" for example seems to fit Stephen's Tram Ride experience. Also the "she" in the song is widely considered to be a prostitute. Also the line "People tell me it’s a sin, to know and feel too much within" reminds me a lot of Stephen's religious anxiety. 

In any case, I highly recommend giving Dylan's song a listen to get the full impact... the whole song sounds like a theme-track to Stephen's street ramblings.