"Percy..."

"Yeah, Thomas?"

"...Am I really Sir Topham Hatt's favorite?"

Percy hadn't really known how to answer that question last night when his friend had asked it, and he still didn't really have a good answer. 

It had seemed so out of the blue when they'd been sitting on the hillside, watching some film rolling on the big screen at what Boulder and Blades had called a Drive-In Theater. A massive tarp with moving pictures projected up onto it, Boulder had turned on a radio in his cab to allow them to hear the sound that was supposed to go with it, but somewhere in the middle of the film, Thomas had sat up from where he'd been laying after being quiet for a while, and asked this question-

-A question that was still swirling in Percy's head by morning.

Despite having been in a frustrated rage the afternoon before, Diesel had made some points. Thomas was referred to as 'The Northwestern Railway's Number One Engine,' and to some, that might only be a reference to the number usually painted on the side of his tender, but was it more than that? Percy hadn't really thought about it. He normally had a number six on his own tender, but that didn't mean that he was Sir Topham Hatt's sixth favorite engine, right?

Diesel had also said that Thomas never got in trouble for anything. That wasn't true, Thomas had been scolded by Sir Topham Hatt several times for reckless or dangerous behavior early in his time on Sodor... But then again, scolding was all that he really got. Other engines were punished for their misdemeanors, such as that time James wasn't allowed to pull coaches until he adjusted his attitude, or when Gordon had tricked Henry into not pulling the Flying Kipper one night and was forced to pull it himself as punishment.

Thomas rarely- if ever- seemed to get such punishments. Sir Topham Hatt would give him a stern talking to, but that was it. Was it because he was young? Percy didn't know.

But Diesel was right about one thing... The Diesel Engines were treated differently than the Steam Engines. Diesel engines were typically assigned down to the quarry, the junkyard, or other places were civilians were limited. They weren't seen by the public the same way the Steam Engines were- rolling up to the stations all shiny and clean, puffing their steam proudly and their whistles echoing across the fields like a bird's song.

Sir Topham Hatt checked in on places all throughout the day, but how often did he acknowledge one of the diesels individually, except to reprimand them for something they did wrong? Diesel said he acted up to be seen, was that the only way to get attention for them? Did they just expect to be looked over?

Percy wanted to find out, if that was the case, he should talk to Sir Topham Hatt when they got back to Sodor... Their Sodor, at least.

Boulder and Blades had said that several times the day before- 'Their Sodor.' Did they mean Sodor in their time, or... was it possible that the Sodor they'd been engines on was not the same one they had been exploring? Percy had set that thought aside, not really certain how to ask about such a thing.

Boulder had opted to let the engines sleep in the next morning, which Percy was eternally grateful for. The last two days had been grueling and had worn him out, despite being excited to see what all his new air mosaic status would bring when he really practiced with it.

Time was relative when you spent that time underground, it was impossible to tell what time of day- or night- it was upstairs, and so when the little green engine was finally pulled from sleep by movement, he was both surprised and not surprised when he was told it was nearly noon.

"Noon...?" He yawned, accepting the little cube of energon James handed him. "Really...?"

"Really." James smiled, "We're gonna be heading out to check out the mines soon. Boulder said he found something in Sir Topham Hatt's office yesterday that might be important."

Percy nodded, sipping the energon in the cube and looking around the bunker. The rest of his friends were up, either stretching or refueling like he was. Emily was the one that surprised him the most, however. For whatever reason, the emerald engine was on top of a crate in the corner, balancing not on her feet, but on her hands, her feet folded up over her shoulders.

"Doesn't that hurt!?" Diesel exclaimed as he saw her as well.

Emily looked up, "No? It actually feels kind of nice."

"...That's not right." Gordon muttered, "Engines shouldn't be able to bend like that."

"I'm not an engine right now, Gordon." Emily then folded her legs up and back over, standing up on her feet once more. "Imagine Sir Topham Hatt seeing us doing something like that."

"He'd faint on the spot!" Edward remarked with a smile, "Impressive, though."

"Alrighty, guys!"
Blades appeared from the hallway, Boulder close behind. "Let's get to cracking this mystery!"

"Do you really think the Mines will have any clues?" Diesel asked, "Whay could be down there other than old equipment and rickety rails?"

Boulder grinned, "I found an old note from Sir Hatt that mentioned the mines, not long before the last records of people living there. They found something down there which may help us figure out this mystery. So let's get going!"

The engines nodded, finishing up their energon and then following the two older mosaics through the bunker to the groundbridge.

"What do you suppose it could be, Thomas?" Percy whispered.

Thomas shrugged, "We'll have to see, Percy..."
__________________________

"I didn't like coming here before, but now..."

"Same here..."

Boulder had commanded the groundbridge to let them out onto Sodor within a mile of the mines, and as they'd trudged across the tall grass and hilly landscape, Edward had felt something... off about this area. He saw nothing particularly abnormal, but there was a lingering feeling in the air that felt him shivering and rubbing his arms. He didn't know he could feel cold, but whatever it was, it giving that impression.

Something cold and lifeless, but also telling them to keep out. And Edward wasn't the only one, apparently. The rest of the engines and Cranky are all getting the unsettling feeling, and Blades appeared to be as well, as he looked around every so often as if searching for the source. He would give off soft clicks and chirps, which Boulder replied with his own, presumably some cybertronian language only they knew.

"There, up ahead." Blades pointed, and they reached the top of the hill and peered down to the large, dark hole that acted as the entrance to the mines, the open mouth leading into the deep bowels of the Earth.

"...Yeah, I'm out." Diesel announced, turning around and starting to walk away before Edward grabbed his wrist.

"We do this together, Diesel. Or not at all."

"Then not at all it is!"

"You'd go by this mine on your line, right Henry?" Thomas asked.

The green engine squirmed, "Well yes, but I never had to go in the mine."

"I'm sure it's fine, Diesel." James assured with a nervous grin, "Just a creepy, dark cave."

"You know James, you've never been very helpful. Pretty, but not helpful-"

"Alright alright." Edward gripped the gray engine's arm a little tighter, "The buddy system remains, plus we have Boulder and Blades here. It'll be fine."

Boulder nodded, "Give me a second to check the mine's stability..."
He approached the entrance to the dark space, before clapping his hands and holding them out in front of him.

"What's he doing?" Percy asked.

"Using seismic imaging to check the shaft's structural integrity." Blades explained, "Kind of like how bats use sound to navigate dark places, Boulder can use the vibrations through the rocks to see if any of it is unstable."

"Whoa...!"

After a moment, Boulder motioned for them to approach as well. "Feels stable enough, but everyone stick close, just to be on the safe side."

There was no arguing there. Each approached the hole with caution, Boulder leading the way into the darkness and Blades taking up the rear- one to make sure nobody got left behind, and also to ensure nobody tried to bolt back out without anyone noticing.

Even before they were fully inside, Edward caught the scent of coal. It was still here, there was still plenty of it settled in the walls of this place, which dispelled the theory they'd initially been using. The humans hadn't left because the coal mines ran out, and now that they knew that they left in a hurry, the idea of simply leaving because the industry collapsed also didn't make sense.

The old remnants of a rail line survived in the center of the tunnel floor, urging them to watch their step. A small engine would have hauled the cars of coal out of the mine itself and outside to be loaded into larger cars, which a larger engine like Edward or Henry would have hauled to its destination, but now, the rails were unused, and as they walked deeper, Edward felt the ground squishing a bit underneath their feet.

"It's getting wet down here..." Emily mumbled, sharing Edward's thoughts. "Rainwater must've been pouring in here without anyone to clear it out."

"That just makes things muddy, right?" James muttered, "Ugh, not good for my paint..."

"Not good for the wooden supports either." Boulder informed, "The metal ones supplementing them are good, but they too can rust over time."

"But you said the tunnels are stable."

"At the moment, yes. It would just be something to keep an eye on if we were going to keep exploring these mines for a while."

"So what exactly are we exploring these tunnels for?" Cranky asked, ducking under a particularly low portion of the ceiling.

"The note said that they found a new mineral down here... Something other than coal. If we can figure out what that mineral is, then-"
They all froze as they heard a cracking, and then Boulder looked down. Ahead of them, the ground had been replaced with wooden beams, and with all the water around them... "Okay..."

"What do we do...?" Henry whispered.

Boulder looked over the younger mosaics, and then to the edge of the tunnel. "Everyone carefully move to the edges of the tunnel... The edges are still soil." He tapped his foot again on the ground, "...Looks like it's only for the next twenty meters or so."

"And then we'll go around, right...?" Emily whispered.

"Right. Just move slowly and carefully, and we should be alright."
The engines nodded, following Boulder's lead as he moved to the side of the tunnel and pressed himself against the wall a bit to slide along, feeling the ground under his feet. "...Okay, just step where I step..."

The engines followed the older mosaic along the edge of the wooden gap, pressing their hands against the wall just like he had to keep their balance.

"Ugh... Getting mud in my joints...!" James hissed, "Gross...!"

"It'll wash out." Diesel muttered, "Trust me, I have to get dirty and dusty all day back home. Your splendid paint job will wash."

Little by little, one by one, the cave explorers made their way across the gap, getting to the other side and settling back on solid ground.

The last to cross were the littlest, having decided to let the others go first. After all, if it could support the larger engines' weight, then it would certainly support theirs, right?

But their strides also weren't as wide as the others, and all it took was one step just short of the other footprints when the mud suddenly turned slick.

"Whoa whoa!"

"Thomas!" Percy gasped as his friend slid in the mud and then out of his reach onto the wood, and then another moment before the wood cracked, and the little blue engine yelped as he broke through, disappearing into the dark. "THOMAS!"

"What do we do!?" Emily asked. "We- Edward!"

Edward didn't know what he was doing, but he did know that he was already doing it, running forward and then smashing through the wooden planks himself as he dove down after the little engine, and into the darkness below.