Is coarse but can be polite to those he respects. Barked at Taylor though they were allies at the time, audacity of asking for his help in the middle of a battle and a chance that it could blow his cover in the Merchants. Would likely have gotten along with Rachel.
His personality made him a good fit for the Merchants.
Trainwreck has a round face, small eyes, unwashed greasy hair tied into a ponytail, and acne-scarred cheeks. As Trainwreck, he wears steam-powered armor while his upper face is covered by a metal mask and goggles. Would later receive a gunshot wound. He lacked proper limbs but could shape and mold his body into pseudopods and similar when he was not using a suit. This combined with possible embarrassment over his appearance lead to hygiene issues.
In his power suit he looks a iron giant made of riveted metal with disproportionately large hands even with his large frame. 
Trainwreck is a case 53tinker who needed to build himself a body, lacking proper limbs of his own. He was able to slowly shape and shift his bloblike body into working, if slow moving, pseudopods and could generate energy he could use to power his tinker devices. It is unknown if works powered by him were specially resistant to electromagnetic pulses or similar anti-tinker countermeasures.
He is best described as a "crude" specialty resource combat tinker, who can work with scrap and develop something that works surprisingly well from what should be inferior components. His gear is fairly limited, he is almost incapable of making anything 'fancy', but he can build it fast, build it durable, and he can build it from parts as common as a rusted old car.
In practice Trainwreck largely stuck to using a single set of 'Crude' power armor that he maintained, though he could create other gear and weapons. This suit gave him enough strength and durability to defeat Armsmaster when he caught the protectorate hero by surprise. When he operated as a cape he drove the suit without needing to use proper limbs. His suits limbs do not actually enclose any part of him allowing people like Faultline to sever them with a touch.
Trainwreck is a Case 53 - an amnesiac with physical alterations as well as powers, who woke up in a strange city with no memories. In this warped Tinker's case he would settle in the Trainyards, where he eked out a living as a homeless supervillain, trying to build a non-existent reputation. At some point he was recruited by Coil to act as a mole within the merchants.
After the Undersiders led Armsmaster and Dauntless into a garage, Trickster teleported Armsmaster to Trainwreck. Trainwreck grabbed Armsmaster and slammed him against the hood of a car. Armsmaster managed to fight his way free where he fought against Trainwreck and Genesis until Trickster swapped out his Halberd for a car bumper. Trainwreck grabbed Armsmaster and started beating him against the car.
Trainwreck then tossed a bloody and beaten Armsmaster next to Dauntless. He then examined his fellow tinker's Halberd before crushing it and dumping the remains over the fallen heroes, with nearly a chuff.
Taylor and Lisa were on a mission of mercy that took them to the Merchants festival of blood, where Trainwreck was part of the festivities. When Faultline's Crew staged an ambush to steal valuables from the merchants Trainwreck was held at bay by a new addition to the Crew. Taylor enlisted Tranwreck's help in cracking through barrier's created by Labyrinth of the attacking Crew. He did this quickly after some gripping and returned to fighting members of Fautline's Crew.
↑ 1.01.11.21.3Trainwreck was very similar to the Merchants’ Mush, but without the ability to reintegrate into a human form, and the parts couldn’t necessarily move easily. That said, he was his own power supply. He had tinker knowledge that he used to give his body workable joints and more complicated engineering/weapons. - Comment by Wildbow on Interlude 21.y
↑ 2.02.1Then you get someone like Trainwreck, who doesn't have access to outside ideas. He's a case 53 tinker who needed to build himself a body. That example power 'crude tinkering' I noted before? That's his. He can work with scrap and develop something that works pretty damn well. His gear is fairly limited, but he can build it fast and he can build it with parts from a rusted old car. - Comment by Wildbow on Spaccebattles
↑Trainwreck from Brockton Bay had been both a Case-53 and a tinker, reportedly. The guy had taken the oversized silicone testicles meant for hanging off the back of pickup trucks and incorporated them as an ‘easter egg’ in his power armor at one point, to be revealed if the heavy metal ‘kilt’ of his power armor moved the wrong way – usually when he vented enough steam. No surprise that he’d ended up with the Merchants when everything had gone to hell. I hadn’t interacted with him in any casual capacity, but I was on the fence about whether his demeanor meant he’d be less guarded about talking tech, or if the fact that he’d had to build his Case-53 body made it too personal. - Excerpt from Beacon 8.2
↑ 5.05.1Trainwreck – An unwashed, greasy man who wears a crude, steam powered suit. Essentially a homeless solo supervillain, he is initially based in Brockton Bay’s Trainyards. - Cast (spoiler free)
↑ 6.06.16.26.3I could see the machine-guy’s face, a heavy cheeked caucasian with acne-scarred cheeks and long hair pulled back into a greasy ponytail, the upper half of his face covered in a metal mask and goggles, and now I could place him. He was Trainwreck, a fairly thuggish villain that hadn’t made much of a name for himself. I couldn’t say whether that was a suit or actually his body. For all I knew, he was some sort of coal-driven cyborg, or an unfortunate individual that’d been transformed by his powers much in the same way Newter and Gregor had.
And of course, that left the odd one out, the sea-creature, who could only be Genesis, from the Travelers.
Trainwreck dumped a beaten and bloodied Armsmaster to the ground, beside Dauntless. He took a second to examine the Halberd, which he held in his other hand, and then snapped it in his hands and squeezed the remains in his metal fist. He dumped the resulting debris over the unconscious heroes. - Excerpt from Tangle 6.7
↑ 8.08.1A steel giant with massive hands and a spout on its back that was spewing volumes of gray-black smoke had one hand closed around Armsmaster. Repeatedly, methodically, it slammed Armsmaster against the hood of a car. [...] Armsmaster fought his way out from the giant metal hand, but found himself dealing with not just the machine, but a creature from the black lagoon, replete with crustacean armor and octopus tentacles in the place of arms and a face. He managed to fend them off for a few brief moments, until he swung his weapon at the octopus creature and wound up with a car bumper in the place of the Halberd. He didn’t have a grip on the bumper as it materialized, so he fumbled and dropped it. Before he could recover from his surprise or his lack of a weapon, he found himself caught in the mechanical hand. The steam powered giant resumed his methodical thrashing of Armsmaster against the now-battered car, with the octopus-crab man standing patiently by. - Excerpt from Tangle 6.7
↑Trainwreck didn't have a trigger, but he did end up as a resource tinker. He's a crude tinker, if you want to go by the tinker's handbook. - Wildbow on Reddit
↑*Crude inspiration - no need for special materials, tinkerings are rough edged, ugly, big, but fast to put together from raw materials. Emphasis on gear, weapons, armor, hard/impossible to do anything fancy. - Another comment by Wildbow on Reddit
↑ 12.012.1Trainwreck (Deceased) – Few details have been provided, but Trainwreck has a mechanized suit or mechanized body with enhanced strength. Was working undercover in the Merchants on Coil’s behalf before being killed by the Nine. - Cast Page (In Depth)
↑The hull of their boat scraped against the edge of the building, nearly running over the boat that Grue and Bitch had come in on. All of the lights shut off, and the Merchants descended onto the roof. Skidmark, Squealer, Mush, Scrub, Trainwreck, the telekinetic whirlwind lady with the long hair and one other. - Excerpt from Plague 12.2
↑Shatterbird stepped back from the ruined husk of a massive suit of steaming armor and started flying their way, accompanied by a cloud of bloody glass shards. - Excerpt from Interlude 12
↑I needed a heavy metal suit that could carry a package, and Trainwreck died at an inconvenient time. - Excerpt from Monarch 16.10
↑Or, simplifying it, Bitch gets caught earlier on, pre-Undersiders. She gets therapy, her problems are identified and handled, and she winds up on the side of light. A member of the Boston Wards. When the ABB start going nuts in Brockton Bay, she's among the heroes that volunteer to head into town (helped by the fact that she doesn't have guardians nailing her down in one place, as is the case with many Wards).
The Undersiders probably have another heavy hitter - and Trainwreck is one option for Coil to slot into the team. Über (with Leet joining as a matter of course) would be another.
Butterfly effect at work. Dog Shaped Butterfly? - What if 2
↑The rest of the group remain as Coil's playing pieces on the board, holding their own against Armsmaster's Protectorate with Coil's help. The group alters and takes on Trainwreck (who becomes an uneasy acquaintance of Rachel) as well as some hired mercenaries from Boston and New York. - What if