Author's Notes:
This is a branch of a larger crossover scenario, you can read more about it on AO3, DeviantArt, or Patreon. Credit for Tulip Olsen and the Infinity Train go to Owen Dennis, and this version of Susan Who is owed to Dr. Who and the Daleks by Gordon Flemyng, Milton Subotsky, and of course Roberta Tovey.

“You know, Tulip, it’s fair to say that most of us Earthlings go to other worlds with magic, at least in our native timeframe. But you and I know that science can lead the way just as easily as sorcery! Then again, calling it ‘easy’ depends on who puts in the elbow grease, doesn’t it? Now there’s a funny term. ‘Elbow grease’. Do you know where that phrase comes from? I’m honestly asking, I haven’t a clue!”

“Me neither,” replied Tulip, grinning fondly. “I think it’s something the French thought up, but don’t quote me on that.” The butterflies in her stomach were finally settling. The doctor could talk a mile a minute without saying anything, a sort of white noise technique designed to mitigate the natural apprehension that came with Vortex flight. It had been essential when Tulip first started traveling on board Tardis. At first it had reminded her so much of the Train that she edged towards panic, the expansive space seeming to close in while her palm itched with phantom numbers. But then Susan would start talking and nothing seemed very serious anymore.

She still got nervous after takeoff, but it was clear now that the doctor’s machine wasn’t really anything like the one which had snatched her away with a promise of Oshkosh on a snowy day. There was something grand and hopeful about Tardis, distinctly contrasting the undercurrent of sadness which had seemed to haunt the Train cars. It felt like a home, rather than something meant to get you through a rough patch.

“Anyway, must’ve taken a lot more than elbow grease to put all this together. I still can’t believe you made it in your back yard! And in the Sixties!”

Susan flicked a toggle, and the vibrations running through the room changed key. “Well, I might’ve made it sound simpler than it was. Grandfather would say that it was merely the result of a series of highly fortuitous breakthroughs, but the truth is,” and at this the doctor winked, “We’d never have gotten it off the ground if it hadn’t been for that special spot in Cardiff. He had been hired to investigate local phenomena, you see, and it turned out to be a sort of dimensional rift! In Wales, of all places! We gathered samples of some exotic materials, caught weird energy in specialized batteries, and picked up what I think was a graph calculator from an alien’s pocket, and we were off to the races!” A theatrical swing of her arm, and a lightbulb flickered to life somewhere overhead. “Of course, grandfather and I knew that we couldn’t tell the authorities what we’d really found. Power like that is too potent to trust to anyone with an adult frame of mind. But we couldn’t just sit on our discovery, so we took our souvenirs home, found an old police box in a junkyard, and…. Well! The rest is history.”

“I knew there had to be more to it! Y’know, I’m never going to be able to handle this thing alone. I design video games in my spare time, I can’t master time and space!” Tulip ran a hand across a railing, talking cats and dogs and little round robots coming to mind. “Besides, Tardis doesn’t like me as much as it likes you. You know this thing is alive, right? It gives you stage lighting!”

“Nonsense,” sighed Dr. Who. “It’s just the odd glitch, and anyone who can rewire super-advanced tech like you do has the touch. After all, what are video games if not training wheels for time and space? Little universes in boxes! You’ll get the hang of it, I’m quite certain.” And Susan meant it very sincerely. Tulip was easily the best candidate for the keys, she thought, if only she didn’t insist on seeing her limitations before her potential! But then, the child’s experience with the Infinity Train phenomenon had clearly tempered the immature impulses which had led her to run away from home once upon a time. And if you were going to travel by Tardis, the first thing you needed was an immature impulse to run away from home!

The doctor turned to confirm their landing coordinates on one of her new touch-screen panels, still rather missing the tactile sensations of the previous worn-out switches. But no one would ever accuse her “Whovian” science of being behind the times, especially not now that the Society had a fresh batch of junior members used to their fancy-dancy plastic phone screens. Though there were still a few analog buttons and dials bolted on odd surfaces, distant echoes of the absolute mess of junk that she and Grandfather had initially cobbled together around their stolen bits of extraterrestrial rubbish, hundreds of wires dropping from the ceiling like an exploded string factory. There hadn’t even been a central work station at first.

Nowadays it was important to keep the place tidy for guests, and her fellow Society members were always a tough crowd to impress. Some even had ships of their own, like Frizzle and her silly school bus (a matter of friendly rivalry). And Tulip already had extensive experience with abrupt scenic transitions through transcendental technology aboard the Infinity Train. The two of them had made that particular mystery their pet project, as Susan hypothesized that the locomotive existed in a branch timeline which had weathered an apocalyptic event which the sapient locomotive was programmed to divert by strategically lifting self-destructive humans from the past and rehabilitating them. It may also have been the product of an elusive, extinct alien race that she’d been chasing for many years, the only civilization she’d ever encountered which had mastered the trick of dimensional manipulation like the Who family had. But that was neither here nor there. Literally.

Her mind wandered back again to that jungle of dangling wires, and to Grandfather. The only person in the world who recognized the potential of a humble police box, and of a little girl who was more brilliant than anyone really understood, who never could have flourished without a kindred spirit. She and Tardis were the same in that respect. It seemed right that it would be grown from ordinary things clustered around the odd miracle. To this day she still resorted to scavenging drainpipes and recycled engine parts to keep certain sections of the old girl running, though she had contact with alien vendors from whom she purchased extraterrestrial hardware. She always wanted for there to be bits of Earth in the Tardis, tying it forever to its home. Grandfather would often remind her to value the so-called mundane, the wonder of every scrap of creation. Then he told her stories of a Phantom Tollbooth that had visited him as a child, and begged her to never take anything for granted. Gratitude, he would say, is a secret to living a full life.

She signaled to Tulip. “Take over, would you, dear? You can manage the last leg. Speaking of legs! I have to rest my knees if we’re going to be walking on rocks. And I just know there’ll be rocks! Most planets have rocky ground, you know. Like an endless gravel pit.”

It was quiet aboard Tardis today, no turbulence in the Vortex to speak of. Susan sat in her wooden chair and listened to the hum of her ship, allowing herself to drift off while the Perennial Child began to take over the landing procedure. She closed her eyes and remembered the Daleks, the terrible bio-mechanical fascists whose shrieks still echoed in her ears. She reviewed a hundred close calls on a hundred different worlds, every single day a gift. She recalled her first contact with the Persephone Society, Orithyia Blue (alias Mrs. Hyppolyta Freeman) tracking them down and offering her family a place at the Newcastle. Grandfather had become one of the very few honorary male inductees, augmenting the organization’s technological resources to match its classic mystical wards, and for a while Barbara and Louise and Dr. Who and Susan hosted interplanetary expeditions with all their new friends, one adventure after another. Crossing timelines on missions of mercy like a cosmic coast guard, rescuing the crews of the Spindrift, the Jupiter 2, and the Yonder. She smiled as she revisited the camping trip to post-revolutionary Skaro, telling ghost stories in the petrified forest with Meg Murry O’Keefe. That was as good as it got.

Then one day Barbara and Louise said that they’d had their fill and decided to settle down. Meg became a teacher, and each of them contributed to the world at large in their own special ways, investing what they had learned like subtle seeds in the garden of humanity. Susan couldn’t blame them, really. Except that she did. Because more and more it was just her and Grandfather, who’d never given up his determination to go out and see what the Universe had to offer. There were always members of the Society willing to tag along, but she could sense that her generation was passing its prime.

“Almost there,” said Tulip, quite the perfect little conductor. Susan laid back, letting herself dream. As usual she found herself with Grandfather on their last voyage together. It was meant to be a jaunt around the block while he trained her to fly on her own, but something happened on their way to the moon, pulling them towards the constellation of Kasterborous. Tardis was often temperamental, but this was like skidding off the Ratcliffe Highway and winding up in Australia!

They found themselves on a ghostly world. Desolate, but with signs of a supremely advanced civilization clinging to the dust. How could they resist exploring? And their curiosity was rewarded by signs of transcendental engineering, very much like their Tardis! Proof of a race so great that even their footlockers were bigger on the inside. Grandfather fashioned a knapsack out of one such box and they greedily salvaged bits and bobs from the detritus. For scientists like them it was like a house made of sweets, but they forgot to watch out for the witch.

Grandfather saw it first. A light in the distance, perhaps a sign of life. As they approached the clearing they felt that they were on hallowed ground, and there they beheld something not meant for human eyes.

There aren’t words for the experience. Even after all these years, how do you describe everything all at once? An untempered schism, a gap in the fabric of reality through which they saw the whole of the Vortex, the raw power of time and space channeling limitless data into their simple mortal brains. It felt like a million years before she felt the sand beating under their feet, running as fast as they could under a starless sky.

The elderly Dr. Who passed away on the return trip. He murmured of enlightenment, of tears and of life and of hope, and reminded Susan to keep herself warm. He’d given her a scarf to make sure.

Susan managed to recover by dint of her youth, but she was never the same. From that point forward she swore she could feel the turn of the Earth, the ground beneath her feet spinning at a thousand miles an hour, the entire planet hurtling around the sun at sixty-seven thousand, falling through space. She knew she would never settle down like Barbara and Louise or her friend Meg. She would always be on the move, flying, running, or walking in eternity. Only Orithyia really understood, and eventually she had to leave as well. Now Susan remains, the oldest active member of the Persephone Society. And for all the pins and needles of longing and loneliness, in her heart she feels nothing but gratitude.

Bump.

“We’re here! Hey, I did it.” Susan’s eyes fluttered open and she grinned to herself, perfectly relaxed.

“Of course you did, dear. I wasn’t slightly afraid. Now, let’s see what’s out there.” She hoisted herself from her chair, double-checked the atmospheric readings, and ran a quick test to see that Mary Sue still had a strong signal tethering them to Earth. She tightened her laces, made sure her scarf wasn’t likely to flap about (terribly hazardous, but she couldn’t bear to do without it), and tuned her auditory analytical aide, a sort of sonic screwdriver she’d managed to cobble together from a 23rd century refugee’s tuning fork and the keepsakes she’d scrounged on that ghostly planet. She still hoped to find that place again someday, maybe see if it had any ties to Tulip’s train, but it would have to be soon. She couldn’t keep this up like she would when she was a sprog.

Tulip could hop right out the doors in an instant, but she waited for the old woman to lead the way, bless her heart. Yes, thought the doctor, it would have to be her, the impossible girl with no reflection. All the new kids were wonderful, but only Tulip was cut out to cover the more scientific side of cosmic exploration. She’d probably have a doctorate of her own one day, and perhaps then Susan could give herself the quiet life she’d always been after (believe it or not).

For her part, Tulip bounced on her heels as the doctor stepped out and an alien breeze wafted inside like fancy perfume. The truth was, she thought she probably could manage Tardis on her own, but she didn’t want for Susan to think that she didn’t need her close by. She imagined Amelia, a tin god covered in numbers, as full of regret as anyone could ever be, probably still trapped on that Train to nowhere. And now here Tulip was on the other end of the spectrum, with someone who was so full of hope and freedom. Maybe it was selfish, but she aimed to travel with the woman for as long as she could, so she would always know what it felt like to run away from home the right way.

In the back of her head she had an idea that Dr. Who could go on forever. It was a shame that she only had one life to live.