Over the next couple weeks, Tanjiro, Inosuke and Zenitsu's injuries finally healed and Susamaru initially thought that they would start going on missions together again; however, this was not the case as they all needed rehabilitation training first. She never really understood all the exercises and stretches they had to do but she tried to encourage them nonetheless. Especially since Inosuke and Tanjiro found the training to be very tiring. Eventually the three slayers managed to regain their physical fitness but their training wasn't over as Shinobu wanted to teach them something called "Total Concentration Breathing Constant". This sounded like gibberish to Susamaru but she saw how difficult the training was. So difficult in fact that Inosuke and Zenitsu gave up and Tanjiro became the only one to train for a while.

Zenitsu and Inosuke's continual absence from training eventually grew irksome to Aoi and both Tanjiro and Susamaru could tell it was grating on her nerves. It was at that point Susamaru decided to step in. She calmly approached Zenitsu and Inosuke's beds as they slept in during the mid-morning. She then proceeded to shake them both awake with all six of her arms. Once she had their attention she said, "Alright listen up! I have no idea what the Hell this whole breathing thing is but Tanjiro is working his ass off everyday to master it while you two lounge around! You are going to drag your asses into the training room right now or…" Susamaru trailed off as she raised her hands together and began cracking all of her knuckles. This was enough for the two boys to get the message and Aoi was secretly glad when they began showing back up for training.

It was not long into breathing training when Tanjiro's crow arrived to inform him that he and Inosuke's new swords were arriving. Tanjiro was elated by the news and he quickly went to the training hall to tell Inosuke. Susamaru was watching her strange boar friend as he balanced on top of a broom when Tanjiro burst in.

"Hey, Inosuke! Our new swords are here!" Tanjiro declared with a huge smile.

"Wait, for real?!" Inosuke said in happy disbelief.

"Yes! Let's go!"

"Wahoo!"

"Have fun you guys!" Susamaru waved as the two bounded off to get their new weapons. She was just happy that they had something to be so excited about and she also hoped that this meant they would soon be off on new adventures instead of being confined to the nice but dull mansion. Her thoughts were interrupted by screaming from outside; however, and she sat up to listen to what was going on. She couldn't exactly make out what was being said but she heard lots of running and something like a knife being brandished. She was very confused and time like these were when she really wished she could go outside during the day just so she could see what the Hell was going on. Perhaps fortune decided to smile upon her that day, as soon she became privy to what was transpiring. As she listened for more information, Tanjiro suddenly ran into the training hall at full speed followed by a strange man wearing a Hyottoko mask. Said man was chasing Tanjiro with what appeared to be some kind of kitchen knife and Susamaru immediately jumped on him to pin him down. She used her many arms to lock his head, arms and legs from moving and asked what was going on.

"Thanks Susa! This is Haganezuka. He's the swordsmith who forged my sword… he's not really happy with me right now," Tanjiro said sheepishly.

"YOU BROKE MY PRECIOUS SWORD!" Haganezuka screamed as he flailed against Susamaru with all his strength. Even with her extra limbs and demonic strength, Susamaru was having a difficult time holding this strange man down. She would have commended him for being so strong if he weren't clearly insane.

"I'm so sorry!" Tanjiro apologized as he bowed repeatedly to the swordsmith.

"Don't apologize to him! He's trying to stab you!" Susamaru said as a vein popped out of her forehead.

"But he's right! I was too weak and because of that I broke my sword!"

"Or maybe he just made you a crappy one!" Susamaru said. This struck a nerve with Haganezuka who shifted his attention to her instead of Tanjiro.

"How dare you insult my smithing skills! I'll filet you for that!" Haganezuka yelled as he tried to stab at Susamaru with his knife. Luckily for everyone involved, another swordsmith arrived to defuse the situation and calm his peer down. He then introduced himself as Kozo Kanamori; the maker of Inosuke's blades.

Once introductions were out of the way, Kozo revealed Inosuke's swords to him and stated his pride in them as they changed colors in Inosuke's hands. Susamaru was awed by the display as the swords changed to an indigo gray. She couldn't help herself from asking if she could get some of her own.

"Woah! Can I get one of those?!"

"I'm afraid not Ms. Susamaru. These blades are reserved for demon slayers only," Kozo answered politely.

"Awww," Susamaru pouted.

"My apologies miss. Anyway, Master Inosuke how do you like your new swords? How do the hilts feel in your hands?" Kozo asked as he turned to Inosuke. He didn't show it but he was desperate for feedback on the swords since it was his first dual wielding project and he wanted to ensure both blades were of equal quality to any other he had forged.

Instead of replying, Inosuke merely stood up and walked outside with his blades. Everyone was curious as to what was happening until the boar mask wearing man decided to start chipping his own swords with a rock to create his signature jagged edged blades. Kozo was immediately infuriated by this and leapt outside to go pummel Inosuke. Tanjiro tried to stop him; however, Susamaru held him back as she understood this particular swordsmith's agitation.

"Susa! We have to stop him from hurting Inosuke!" Tanjiro pleaded.

"No, he deserves it… like he usually does," she replied. "Besides, if he doesn't face consequences then he'll never learn things."

"I… guess that's true," Tanjiro said as he sat next to his friend.

"Then why did you pin me down!" Haganezuka said as he brought his face right next to Susamaru's and began twisting his finger into her cheek.

"Ow! Because Tanjiro broke his sword in a fight! You can't get mad at him for that! Inosuke… broke his on purpose just now," Susamaru explained. Unfortunately for her, she did not know that Haganezuka was very bad at listening and didn't actually respond to her explanation. Instead he just kept twisting his finger into her face until he blew off enough steam to become calm once more. It took a bit longer for Kozo to tire himself out from chasing Inosuke and eventually the two slayers and their demon friend bid farewell to the two miffed swordsmiths. Shortly after, the group received word that they would be assigned their next mission.

"Finally! We get to leave the mansion!" Susamaru exclaimed as she jumped in the air.

Her friends were equally eager to leave. They had all finally mastered Total Concentration Breathing Constant and they wanted to put their new strength to the test. Additionally, Tanjiro was excited that they would get to work alongside the Flame Hashira Rengoku. He was hoping the Pillar could enlighten him on his family's own breathing technique. He expressed this to Susamaru who was happy for him but still didn't understand what the big deal with "breathing techniques" was. Regardless, the group packed up their things and headed towards the Mugen Train, where many people were disappearing. They all said their goodbyes to the girls at the Butterfly Mansion and even Giyu arrived to see them off. After that, Tanjiro slung Nezuko on his back while Inosuke slung Susamaru on his. Tanjiro was happy his friend had offered to take her so that he didn't have to carry two people around anymore and Susamaru was happy because she now had her own box like Nezuko's. Tanjiro had written back to his old master Urokodaki to explain that he needed another box for her and his teacher had gladly sent him another. Now that their traveling group had maximized its efficiency, they had no trouble reaching the train just as the sun set which allowed Susamaru to exit her box as they walked up to the train.

"W-what is this creature?!" Inosuke yelled as he approached the train engine.

"It's a train…" Zenitsu said.

Inosuke quickly silenced Zenitsu with his palm before stating that he was going to attack the train. Tanjiro encouraged him to wait.

"This could be the guardian spirit of this land so we shouldn't be attacking it!" Tanjiro explained.

Susamaru was speechless at first. Did these two really not know what a train was? She looked to Zenitsu as he also had a baffled expression on his face. He also couldn't believe that neither of his fellow slayers knew what they were looking at.

"Guys… it's a train. You know? A machine that takes people across the country," Susamaru explained.

"Oh really? It's a machine?" Tanjiro said. He had heard of such things but had never seen a mechanical device growing up.

"Yeah! In fact, you saw a smaller version of it called a trolley in Tokyo!"

"I did?!"

"Yup and you saw cars too. Those are all transportation machines," Susamaru continued explaining. She had been living in Tokyo for a few decades before Tanjiro had saved her and as a result she was up to date on all of the latest human technology. Unfortunately, while Tanjiro seemed to begin to understand what she was saying, Inosuke… did not and proceeded to headbutt one of the train cars much to Zenitsu and Susamaru's shock.

"What the Hell are you doing?!" Susamaru shouted.

"Yeah! Stop embarrassing us!" Zenitsu added.

It was then that the group heard police whistles and turned to see several officers running towards them. The officers noted that the humans had swords and began to chase the group of slayers. Of course, the slayers and Susamaru were much faster than normal humans so they quickly lost the police and found a place in the train station to hide until it was time for them to board the train.

"Why did we even have to run form those guys anyway? What's wrong with having swords?" Inosuke asked as he sat on the concrete.

"Swords are illegal dumb ass. I though you all would know that," Susamaru said.

"They are? But… don't they know we need them to slay demons?" Tanjiro asked.

"No they don't. Normal humans don't even know demons exist so the Demon Slayer Corps is not officially sanctioned. It would be best if we hid our swords for now," Zenitsu replied.

Inosuke chuckled as he quickly stashed his swords behind his back. Susamaru's box actually hid them fairly well and the others were actually surprised Inosuke thought to use it to cover them.

"Huh, I guess that works," Zenitsu said.

"Yup," Susamaru added not knowing what else to say.

The four of them had little time to continue their conversation; however, as the train began to depart and all of them had to scramble to get on board. Inosuke, Tanjiro and Susamaru made the jump onto the train easily but Zenitsu had to be pulled up. Eventually the four of them made their way to the interior of the train where Inosuke would have to be calmed down in order to not start destroying the inside. Once he was calmed, Tanjiro began leading them towards Rengoku as he remembered the Pillar's scent from when they first met.

"Tasty! Tasty! Tasty!"

The group paused as they saw a flame haired man shouting "tasty" every time he took a bite of his meal which was apparently substantial as evidenced by the many empty food boxes being carried off by the train's staff.

"Is this guy seriously a Hashira? I mean… I know he is because of the meeting but… he's really… weird. Then again… so are most of the humans I've met over the last few months," Susamaru pondered as she and the others walked over to meet up with Rengoku. Tanjiro immediately began asking the man about the dance his family had taught him; however, the Hashira proved to be of little help and instead redirected the conversation towards the other breathing techniques and wanted Tanjiro to learn his flame breathing style. As he was engrossed with talking to Tanjiro, Inosuke attempted to leap out of the train so he could "race" alongside it. It took the combined effort of Zenitsu and Susamaru to stop him from enacting his crazy plan. Rengoku interrupted the three of them and revealed that the demon they would be facing was actually aboard the train. This caused Zenitsu to panic and let go of Inosuke; meaning that Susamaru now had to keep trying to pull him back onto the train by herself. She was tempted to simply knock Inosuke out for the rest of the mission to get him to cease his madness.

Before she could act on that impulse, the train conductor arrived to punch their tickets. He seemed very strange to Susamaru; as if he were sick or something. He spoke in a very tired manner; his movements were slow; and his posture was slouched. He also had dark circles under his eyes which stared into the distance. She found him a bit unnerving but she presented her ticket to him anyway. Not long after that, Susamaru watched him as he slowly shuffled into the next car.

"All of a sudden this Rengoku guy doesn't seem as weird," she thought as she turned to ask Zenitsu for his opinion on the conductor only to find he was asleep. "Typical… he always falls asleep when he's scared." Susamaru then turned towards Inosuke who she found was also asleep. This was puzzling to her. He had boundless energy a few seconds ago. Why was he out cold now? She then gazed around the train and saw that everyone in the entire car was sleeping including Rengoku and Tanjiro. Now she was really starting to suspect something was up so she got out of her seat and tried to shake Rengoku and Tanjiro awake. She shook them as hard as she could without hurting them and she even tried slapping them but nothing worked. They were dead to the world and it was starting to really worry her. It was then that she caught the scent of another demon coming from the window Inosuke had opened earlier. She immediately jumped towards it and crawled out of the train towards the roof. Once on top, the scent became stronger and she followed it to the front of the train.

When she arrived at the front she saw a male demon dressed in a fancy suit. He had his arms outstretched to feel the wind against them and appeared to not notice her advance. She quickly sprung up to attack him; however, he turned and quickly swatted her away. She landed onto the train roof with a "thump" that left a dent in the metal. She recovered rapidly and stood back up. Now that he was facing her, she saw the symbol for "one" in his left eye signifying he was the top member of the lower ranks of the Twelve Kizuki; Lower Moon One.

"How curious. A demon attacking me? What an interesting turn of events," the demon said as he brought his hands to his face which was disfigured by an excessive amount of veins that bulged across his whole body. "It's good to know my Blood Demon Art doesn't work against fellow demons for when I challenge one of the Upper Ranks at some point," the demon continued to talk; however, his speech was more directed towards himself than Susamaru.

"So it was your Blood Demon Art that put everyone on the train to sleep?" Susamaru asked. She was trying to buy some time to develop a strategy against her new opponent. Unfortunately, she didn't realize he was trying to do the same and thus was happy to indulge her in a conversation while his human minions went to work on destroying the Demon Slayer's spirit cores.

"Why yes! I do so love putting humans to sleep with such sweet dreams… only to then replace them with the most horrible of nightmares. The way their faces twist in agony and fear causes me such great pleasure," the demon said with a blush adorning his face.

"… even by demon standards… you're a creep," Susamaru said with a deadpan expression on her face.

"A 'creep'? Huh… that's a new label I don't think I much like. I think I'll… kill you for that," the demon said.

"How are you going to do that without your Blood Demon Art?" Susamaru taunted. She still had no idea how to fight him herself. He was clearly much stronger than her physically speaking and she doubted her temari would cause any damage to him so she was just hoping he would reveal some sort of weakness to her at this point.

"Oh, like this," the demon said before massive fleshy tentacles sprouted from the train and wrapped around Susamaru. She was caught completely off guard as she expect this to be a more hand-to-hand combat sort of fight due to his Blood Demon Art being ineffective; however, this demon clearly had more tricks than he let on. He then proceeded to chuckle at the surprised expression on her face. "I can see you are wondering how I did that. Just to humor your curiosity, this," he then gestured towards himself, "is not my real body. I have merged with the entire train. Now I can devour all two hundred passengers at once! As for you… I may not be able to give you nightmares… but I will still enjoy your screams of agony as I squeeze and tear you apart over and over and over again for trying to get in my way. I was going to ask you why you would try to stop me but… I realized I'd enjoy your sorrow and pain much more than any explanation!"

"Shit! I can't get free!" Susamaru thought as she struggled against her flesh bindings. She felt as if she were a child trying to wrestle an adult. Now she was powerless and this monster was going to devour every human on the train including her friends. Her stomach sank as she realized her situation and seeing her fear only encouraged the demon before her more as he began cackling at her expense. He then began squeezing her which caused her joints to start dislocating and her bones to start cracking. It was very painful; however, it wasn't as painful as what she had endured before. What was painful was knowing that she couldn't save her friends. "Tanjiro, Nezuko, Inosuke, Zenitsu… I'm sorry… I'm sorry I couldn't help you. I'm so sorry," she thought as she began to tear up.

Enmu was delighted to see his adversary so utterly hopeless. It thrilled him almost as much as when he watched his master dispatch the other Lower Moons a few months ago. He so enjoyed the suffering of others and he felt like he was living a constant dream… that was until he sensed a human on top of his new train body.

"Let her go!" the human yelled as he cut the fleshy appendages holding Susamaru.

As she fell back onto the train's roof, Susamaru looked and saw that Tanjiro was the one who saved her! He had somehow woken up! A huge smile adorned her face as he went to strike the other demon down. Unfortunately, Enmu began to use his Blood Demon Art to put Tanjiro to sleep once more and Susamaru was worried Tanjiro wouldn't be able to react in time to stop the powerful demon from killing him in his sleep; however, Tanjiro already knew the trick to Enmu's Art and in a move that surprised the Kizuki, Tanjiro was able to use his Water Breathing Style to decapitate the dream demon. Susamaru was about to celebrate before she realized what Enmu had said earlier about that not being his real body and she was quick to point it out to her friend.

"Tanjiro! That wasn't his real body! He's become the whole train!"

"What?!" Tanjiro spun around to face his friend so he could confirm that he heard her right.

"Why yes, hanafuda boy. You'll have to do better than that to kill me," Enmu's severed "head" said as it began merging with the train. "I now see why HE wants you dead so badly. You really are quite irksome."

"Uh oh, this is bad! Susa! We have to protect everyone on the train! Try to wake up the others!"

"I already did! You guys wouldn't respond to anything!"

"Look out! Lord Inosuke is coming through!" The two friends turned towards the sound of Inosuke shouting. They were happily surprised when they saw him burst through the roof of a train car.

"Inosuke! The passengers! You have to save them!"

"I knew it! I am the great and mighty Lord Inosuke! I shall save EVERYONE!" He screamed as he began slicing through the countless limbs of Enmu that had invaded the train car closest to Susamaru and Tanjiro. They soon jumped inside the train to help him and eventually came into contact with Zenitus and Nezuko who were also defending the various passengers. It wasn't until Rengoku arrived; however, that the group managed to create a solid plan to defeat Enmu. Rengoku encouraged Tanjiro by saying every demon had a weakness even when they didn't have an obvious "neck" and then declared he would protect five train cars while Nezuko and Zenitsu would protect three. He then tasked Tanjiro, Inosuke and Susamaru with slaying Enmu. With her friends alongside her, Susamaru felt much more optimistic about their chances of victory.

She had no idea what it would take for their victory to be achieved unfortunately...