The lines are the main thing
Starting with a stutter-step stumble, tumbling into stolid consciousness. Twilight shakes her head and falls awake as the train lurches to a stop. Pulling her luggage behind her, she lurches from one sleeping hoof to the next as she emerges from the train and into the daylight of the station.
A few other ponies pass her by as they make there way through the newly opened air. A half-remembered lozenge--she must have fallen asleep with it in her mouth--rolls around her molar.
The crude wooden schedule confirms her one-hour layover as the train behind her starts up again. The wheels roll and screech, inseparable movement marked by the individual wooden slats beside, so you could almost fool yourself into believ--
Ugh, that again. Stop it. Twilight shakes her head again and winks each eye separately, forcing herself to focus. A few wooden homes cluster around the station and a small hotel. With renewed resolution, Twilight sets off for the hotel and the promise of some coffee to clear her head.
The sign at the front of the cafe declares, "Seat your elf!" to everyone who enters. Everyone including her as she sucks on her menthol flavored candy and walks past everyone in their booths and tables. Chewing mouths and clicking silverware and dripping water beading from the sides of glasses.
Despite over a year of friendship lessons, she still has the one object in mind. A small booth in the back corner. There is always such a booth, and it will always be empty, and she will always have a quiet place to sit. This is the sort of Universal law that you can build an identity around as she heads toward the empty booth.
Not empty.
Twilight stifles her frustration as she goes to look for a second.
Wait a second. Something she almost noticed tickles her hind brain, or perhaps something has been tickling her hind brain and she's just now almost noticing it. A trickle of sweat slides down the back of her neck.
Twilight stifles her cough. She swallowed her cough drop. When? It must have--
Stop it. You've dragged this out long enough. Twilight Sparkle is still sitting in the booth as she turns back to face herself. She smiles at herself, her luggage already sitting in the booth beside her, and a flashy top hat resting on her head, and a book open in front of her, and she is sitting there staring at herself as if it were the most normal thing in the world to meet yourself in a cafe.
"Hello."
"Hi," she croaks at the image in the booth.
"Would you like a seat?" She gestures across from herself at the empty half of the booth. "My name is Twilight Sparkle."
How is she so calm about this? Maybe I'm still asleep? Twilight slides into the booth across from herself. She tucks her luggage under the seat. Just like she's apparently already done when she got here before herself.
"Would you mind introducing yourself, Twilight?"
"But if you already know … why should I ..."
"Manners. Manners are important," Top hat Sparkle chides her hatless companion. She shouldn't be wearing a hat indoors, that's manners. And just who is she--who am I, maybe--to show up and start lecturing myself about--
"I'm Twilight Sparkle," she cuts off her own internal monologue. Talking to yourself won't get you anywhere, talk to … your other self.
"Nice to meet you," Twilight nods at the newly arrived Twilight.
She feels cramped, even though a yellow placard advises that this booth has room enough for four friends. It is rather specific on friendship being a requirement for this capacity, and Twilight can't help but wonder how many of the same pony it is rated to contain.
Probably they don't often get a chance to test that sort of thing.
The Twilight who was already there is quietly sipping on her cup. As she turns the pages of the book she is reading, the newly arrived Twilight can recognize the scribbled notes--her scribbled notes--in the margins.
"How did you get here?" she finally asks herself.
"By train. Don't tell me that you walked?"
The only roads to take hoof traffic out of here are a few dirt ones. This entire town born just by a fluke of two train lines needing somewhere to shuffle ponies back and forth. A wagon bounces back and forth as it passes by the window.
"This shouldn't be possible."
"You mean to say, it isn't probable," Top hat Sparkle rearranges herself--she seemed to be having difficulty balancing the hat against her horn without losing it--and Twilight was sure she didn't normally dress that way. At least, she knew that she didn't often dress that way, but she knew she also did not accost herself in strange cafes. "It is happening, so that makes it possible. Necessary even."
Twilight bites her lip. The waitress slides by their table, apparently oblivious to the paradox before her. She takes Twilight's order and doesn't bat an eyelid as she takes the other Twilight's order for a refill of the same.
Twilight continues to explain herself to herself. "If I may site the short essay I wrote for Celestia, on the possibility of extraterrestrial life? In an infinite Universe over an infinite period of time, the only barrier to something occurring, such as intelligent life arising on a planet, is that this intelligent life should arise once. After the point is proven, for example by one planet with intelligent life or one mare who is Twilight Sparkle, then it is only the most ridiculous egotism that insists it can only happen once. In fact, if it has happened once, it must be happening an infinite number of times."
"I wasn't talking about myself-"
"Yes, I was. Everyone only ever talks about themselves."
Twilight narrows her eyes. "That's from my journal when I was 14. It was a difficult year for me."
"I started listening to Dummy Mare," Twilight agrees with herself. Then catches herself and shakes her head.
The waitress returns, setting steaming mugs in front of each Twilight. Two pairs of identical sugar cubes, fallen from some manufacturing line somewhere, fall into the cups and are stirred by two identical spoons.
Without thinking the Twilights take identical sips, as if the whole thing were choreographed in advance.
"You don't believe me?"
"About what?"
"You're pretending not to understand me?"
"Would you understand?"
"I didn't, so I won't expect you to." Twilight turns back to the book she'd been reading before Twilight had arrived.
"What do you mean didn't? Are you from the future?" There was a lever on this, Twilight was certain. If she could just get hold on it somehow, the matter could be moved into somewhere sensible.
"Of course not."
"A parallel Universe?"
"Don't be absurd. There is only one Universe. There is only one history. There is only one calculus. There is only one meter. Anything that is important, there is only one of. Everything meaningless is infinitely reproduced according to the singular."
The clock outside turns toward two o'clock. It is almost time for the next train.
They stood together on the platform, waiting for their trains to arrive, taking them in opposite directions. The Twilight wearing the hat tapped the side of it with her hoof.
"You never asked me about this. Rarity made it for me."
"So you have a Rarity, too?"
"Not A. Rarity, she's just Rarity." Twilight smiles slyly at her gag, "no first initial."
"I meant a Rarity," Twilight ground out the article.
"Don't be vulgar." Twilight shook her head. "She is Rarity. That's her name."
"Yes, but, if there are more than--"
"Are you a Twilight?" The other cut in.
"I," Twilight looked down, "that's how it seems."
"If we had never met, would you be any less or more yourself? Would I exist any more or less?"
She couldn't take it any longer. "How are you so calm about this?"
"Because it happened to me on my way out. You'll be calmer too, I suppose, when this happens on your way back."
"How do you know I'll even get out of the train? Or that I won't take a different route?"
"I couldn't resist the urge of an experiment. To prove someone wrong, even if I'd never see them again. And if I'm not wrong, if I do meet myself, then how could I resist the urge to lecture me? To regain some measure of control against all this?"
"And you'll be here?"
"No, another. An infinite number spread throughout an infinite space over an infinite amount of time, parallel lines meeting just once."
Twilight waited another minute. Let another train pass with its heart shaped windows reflecting the world in double; let her own heart beat quietly in trepidation. "So that's your theory?"
"Yes."
"Would you like to hear mine?"
"Of course."
"Someone is screwing with me."
Twilight smiles. She laughs. Her laughter comes in short bursts. Hyenas with machine guns in their throats. Her head tips back. The hyenas are excited and charging and machine gun laughing. Reality rumble-tumbles down the slope toward and upon her. Her hat--awkward top hat awkwardly tipped--tips off and pitches down between them. There came a loud whistle blast and a gout of steam passed around them. There went the world turning white and strange and vanishing in a tide of alien winter.
When the outside world returned, the top hat was nowhere to be seen, and it was impossible to tell which Twilight was which.