OPINION OF THE COURT

HARDIMAN, Circuit Judge.

Peggy Hassell, on behalf of herself and her deceased husband's estate, appeals an order of the District Court dismissing her civil suit against The Budd Company and Resco Holdings LLC. Hassell asserted state law causes of action arising from her husband's exposure to asbestos during the forty years he worked for the Atchison, Topeka and Santa Fe Railway (the Railroad). Budd and Resco moved to dismiss, arguing that Hassell's claims were preempted by the Locomotive Inspection Act, 49 U.S.C. § 20701 et seq., and the Safety Appliance Act, 49 U.S.C. § 20301 et seq. The District Court granted the companies' motion, holding that Hassell's claims were preempted by the Locomotive Inspection Act.

In this appeal, Hassell claims that the District Court erred procedurally by dismissing her complaint based on facts that were neither in her complaint nor undisputed. Hassell also contends that the District Court misapplied the preemptive scope of the Locomotive Inspection Act to hold her claims preempted. Because we agree with Hassell's procedural argument, we will vacate the Court's order and remand the case for further proceedings consistent with this opinion.

I

Toward the end of the nineteenth century, the rapid growth of the railroad industry in the United States brought with it numerous accidents and deaths. See, e.g., Charles W. McDonald, Federal Railroad Administration, The Federal Railroad Safety Program 2-6 (Aug. 1993). In response to these safety concerns and because of the variety of state laws regulating the industry, Congress in 1893 passed the Safety Appliance Act (SAA). Act of Mar. 2, 1893, ch. 196, 27 Stat. 531-32, amended by Act of Mar. 2, 1903, ch. 976, 32 Stat. 943, and Act of Apr. 14, 1910, ch. 160, 36 Stat. 298; see also Lorenzo S. Coffin, Safety Appliances on the Railroads, 5 Annals of Iowa 561, 569-80 (1903). Full implementation of the SAA, which required railroads to equip trains with automatic couplers and power brakes, was delayed until 1900. See Note, The Federal Safety Appliance Act as a Regulation of Interstate Commerce, 3 Mich. L.Rev. 387, 388 (1905). Eleven years later, Congress began regulating locomotive steam boilers through the Boiler Inspection Act (BIA). Act of Feb. 17, 1911, ch. 103, § 2, 36 Stat. 913-14. Soon thereafter, the BIA was amended to cover the entire locomotive as well as its "parts and appurtenances." Act of Mar. 4, 1915, ch. 169, 38 Stat. 1192. The statute as amended has since been known as the Locomotive Boiler Inspection Act, or simply the Locomotive Inspection Act (LIA).1

The increased federal regulation of the locomotive industry resulted in conflicts with various state laws. Accordingly, in Napier v. Atlantic Coast Line Railroad Co., 272 U.S. 605, 47 S.Ct. 207, 71 L.Ed. 432 (1926), the Supreme Court was presented with constitutional challenges to laws in Georgia and Wisconsin that required the Court to decide whether Congress intended "to occupy the entire field of regulating locomotive equipment." Id. at 611, 47 S.Ct. 207. The Court noted that the SAA, which included specific requirements, and the BIA, which regulated only boilers, did not preempt the field. Id. As amended in 1915, however, the LIA included a "general" power that "extend[ed] to the design, the construction, and the material of every part of the locomotive and tender and of all appurtenances." 272 U.S. at 611, 47 S.Ct. 207. The "broad scope" of this "general" authority led the Court to conclude that Congress, in enacting the LIA, had "occupied the field of regulating locomotive equipment." Id. at 607, 613, 47 S.Ct. 207. For that reason, "[b]ecause the standard set by the [Interstate Commerce] Commission must prevail, requirements by the states are precluded, however commendable or different their purpose." Id. at 613, 47 S.Ct. 207.2

Almost a century later, the Supreme Court revisited the LIA's preemptive scope in Kurns v. Railroad Friction Products Corp., ___ U.S. ___, 132 S.Ct. 1261, ___ L.Ed.2d ___ (2012). Unlike Napier — which involved the preemption of state statutes — Kurns considered whether the LIA preempted state causes of action. The plaintiffs in Kurns asserted state law defective-design and failure-to-warn claims against the manufacturers of locomotive brake shoes and locomotive engine valves that contained asbestos. 132 S.Ct. at 1265. Underscoring that "Napier defined the field pre-empted by the LIA on the basis of the physical elements regulated," the Court held that the state law claims were preempted because they were "directed at the equipment of locomotives." 132 S.Ct. at 1269 (emphasis added); see also id. at 1270 (Kagan, J., concurring) ("According to Napier, the scope of the agency's power under the [LIA] determines the boundaries of the preempted field."). The Court thus rejected the distinction between common law claims and positive law enacted through state legislation or regulation, holding that Napier's "categorical conclusion admits of no exception for state common-law duties and standards of care ... [because] state `regulation can be ... effectively exerted through an award of damages.'" Id. at 1269 (quoting San Diego Bldg. Trades Council v. Garmon, 359 U.S. 236, 247, 79 S.Ct. 773, 3 L.Ed.2d 775 (1959)). And by holding the plaintiffs' failure-to-warn claims preempted, the Court also precluded the attachment of state law duties or conditions to locomotive equipment because such legal requirements would "inevitably influence a manufacturer's choice whether to use that particular design." Id. at 1268 n. 4.

II

Having summarized the law of field preemption under the LIA, we turn to the parties' dispute in this appeal. Hassell's civil action against Budd and Resco was filed in Texas state court. The case was removed to the United States District Court for the Southern District of Texas and transferred to the Eastern District of Pennsylvania as part of a multidistrict litigation. Hassell then filed an amended complaint asserting state law products liability claims based on the following facts.

Between 1945 and the mid-1970s, Hassell's husband Billie was employed as an electrician by the Railroad. Billie's responsibilities included the maintenance and repair of passenger railcars designed and manufactured by Budd's and Resco's predecessors in interest. Steam pipes running underneath those railcars were insulated with material containing asbestos, and he was exposed to asbestos contained in the dust produced during the maintenance and repair of the railcars. As a consequence of this exposure, Billie contracted asbestosis and mesothelioma. He died on May 30, 2009, during the pendency of this lawsuit.

Budd, joined by Resco, moved the District Court to dismiss Hassell's amended complaint, arguing that her state law claims were preempted by the LIA, the SAA, and the Federal Railroad Safety Act (FRSA), 49 U.S.C. § 20101 et seq. The District Court denied the motion without prejudice in light of our intervening opinion in Kurns v. A.W. Chesterton Inc. (Kurns I), 620 F.3d 392 (3d Cir.2010), and the Supreme Court's decision to grant a petition for writ of certiorari to hear that case. See Kurns v. Railroad Friction Products Corp., 563 U.S. 1032, 131 S.Ct. 2959, 180 L.Ed.2d 244 (2011). By this point in the proceedings, the parties had already completed substantial discovery.

In February 2012, the Supreme Court affirmed our judgment in Kurns I and Budd renewed its motion to dismiss (which Resco again joined). See Kurns, 132 S.Ct. at 1264. In the company's renewed motion — which it "[a]lternatively" styled as a motion for summary judgment, App. 37a — Budd observed that the Supreme Court in Kurns had reaffirmed the scope of LIA preemption as defined in Napier and argued that the LIA preempted Hassell's claims because the asbestos-insulated steam pipes on the passenger railcars qualified as locomotive "parts and appurtenances" under the statute. 49 U.S.C. § 20701. Budd claimed that the "pipes were connected to the locomotive, which supplied heat from the locomotive's engine to the pipes," and that "[t]his kind of interconnected system qualifies as an appurtenance of the locomotive." App. 49a. Hassell countered that Budd had produced no evidence to support the company's factual allegations, and that, in any event, she had produced evidence that the Railroad had used "power cars" to heat passenger compartments,3 such that the passenger railcars "would not even have [had] a metaphysical connection to a locomotive." App. 85a. Hassell's argument therefore distinguished between "locomotive appurte-nances" — to which she conceded LIA preemption applied — and non-locomotive equipment on passenger railcars, which she contended did not raise preemption concerns. See App. 79a.

After hearing argument from the parties, the District Court granted Budd's renewed motion to dismiss Hassell's amended complaint. In doing so, the Court began by construing "parts and appurtenances" under the LIA based on Southern Railway Co. v. Lunsford, 297 U.S. 398, 56 S.Ct. 504, 80 L.Ed. 740 (1936). In Lunsford, the Supreme Court defined "parts and appurtenances" under the LIA as encompassing "[w]hatever in fact is an integral or essential part of a completed locomotive, and all parts or attachments definitely prescribed by lawful order of the Interstate Commerce Commission." Id. at 402, 56 S.Ct. 504. Noting that it had previously found railcar brake shoes to constitute "parts and appurtenances" under the LIA because "they [were] part of the interconnected locomotive braking system" in Perry v. A.W. Chesterton, Inc., 985 F.Supp.2d 669, 675 (E.D.Pa.2013), the District Court found that the pipes responsible for Billie's asbestos exposure formed a "system of pipes that connect the railcars and locomotives, which are an essential and integral part of the completed locomotive." App. 7a. Accordingly, the District Court held that Hassell's claims were preempted under the LIA. Hassell filed this appeal.4

III

The District Court had diversity jurisdiction to adjudicate Hassell's state law claims under 28 U.S.C. § 1332(a) because Hassell is a citizen of Texas, Budd and Resco are incorporated and have their principal places of business in states other than Texas, and the amount in controversy exceeds $75,000. See Hertz Corp. v. Friend, 559 U.S. 77, 85, 92, 130 S.Ct. 1181, 175 L.Ed.2d 1029 (2010); Johnson v. SmithKline Beecham Corp., 724 F.3d 337, 347 (3d Cir.2013). We have appellate jurisdiction under 28 U.S.C. § 1291.

Our review of the District Court's dismissal of Hassell's amended complaint under Federal Rule of Civil Procedure 12(b)(6) is plenary. See, e.g., Great W. Mining & Mineral Co. v. Fox Rothschild, LLP, 615 F.3d 159, 163 (3d Cir. 2010). In reviewing whether Hassell stated a viable claim, we must accept as true all plausible facts alleged in her amended complaint and draw all reasonable inferences in her favor. See, e.g., Connelly v. Lane Constr. Corp., 809 F.3d 780, 786 n. 2 (3d Cir.2015). We review the District Court's formulation and application of the test defining the scope of the LIA's field preemption de novo. See, e.g., Addie v. Kjaer, 737 F.3d 854, 865 (3d Cir.2013) (citing Fed. Kemper Ins. Co. v. Rauscher, 807 F.2d 345, 348 (3d Cir.1986)).

IV

A

Napier and Kurns establish that field preemption under the LIA turns on one fundamental question: is the state regulation or cause of action "directed at the equipment of locomotives"? Kurns, 132 S.Ct. at 1268. If it is, the regulation or cause of action is preempted because it falls within the regulatory space assigned by the statute to the Federal Railroad Administration. See Kurns, 132 S.Ct. at 1269; see also, e.g., Del. & Hudson Ry. Co. v. Knoedler Mfrs., Inc., 781 F.3d 656, 659 n. 2, 661-62 (3d Cir.2015); Oglesby v. Del. & Hudson Ry. Co., 180 F.3d 458, 460 (2d Cir.1999) (per curiam); Union Pac. R.R. Co. v. Cal. Pub. Utils. Comm'n (CPUC), 346 F.3d 851, 869 (9th Cir.2003); Springston v. Consol. Rail Corp., 130 F.3d 241, 245 (6th Cir.1997); Mo. Pac. R.R. Co. v. R.R. Comm'n of Tex. (MOPAC), 833 F.2d 570, 576 & n. 7 (5th Cir.1987); Marshall v. Burlington N., Inc., 720 F.2d 1149, 1152 (9th Cir.1983).

Neither Napier nor Kurns had to determine precisely which mechanical components of a train qualify as the "equipment of locomotives" because the answer was obvious in both cases. At issue in Napier were state statutes requiring railroads to install cab curtains and automatic doors in locomotives. 272 U.S. at 208, 47 S.Ct. 21. Kurns involved the imposition of state standards of care regarding locomotive brake shoes and engine valves. 132 S.Ct. at 1264. Thus, neither case had to confront the distinction between locomotive equipment and equipment belonging to some other railroad apparatus — in this case, passenger railcars. Nor are we aware of any other federal court of appeals' decision that has had to squarely confront this distinction.

In the absence of clear guidance on the issue — and perhaps justified by a comment we made in Kurns I, 620 F.3d at 396 n. 5 — the District Court relied on Lunsford, in which the Supreme Court construed the term "parts and appurtenances" under the LIA to encompass "[w]hatever in fact is an integral and essential part of a completed locomotive, and all parts or attachments definitely prescribed by lawful order of the [Federal Railroad Administration]." 297 U.S. at 402, 56 S.Ct. 504. Although Lunsford is not a case about preemption, see note 5, infra, the implicit logical chain between Napier and Lunsford inferred by the District Court is clear. The authority delegated by the LIA permits the Federal Railroad Administration to regulate "the locomotive ... and its parts and appurtenances," 49 U.S.C. § 20701, and those "parts and appurtenances" include anything that is "integral and essential" to the "completed locomotive," Lunsford, 297 U.S. at 402, 56 S.Ct. 504. Therefore, because the scope of regulatory authority delegated by the LIA is coextensive with the scope of field preemption under the statute, the District Court reasoned that any state regulation of an "integral or essential" locomotive component is preempted. See Kurns, 132 S.Ct. at 1268-69; see also Napier, 272 U.S. at 611-13, 47 S.Ct. 207.5

B

As a necessary predicate to its conclusion that the asbestos-insulated pipes on the passenger railcars manufactured by Budd and Resco were "integral and essential" to a locomotive, the District Court found that the pipes were joined to create a "system of pipes that connect the railcars and locomotives," and that this system was "an essential and integral part of the completed locomotive" under Lunsford. App. 7a. These conclusions cannot be squared with Hassell's amended complaint, however, devoid as it is of any facts establishing a "system of pipes" connecting the railcars to the locomotive. As Hassell observes, the word "locomotive" never even appears in her amended complaint.6 Thus, the District Court necessarily relied on evidence extrinsic to her pleadings to grant Budd's motion.

This was error because "a court considering a motion to dismiss under Federal Rule of Civil Procedure 12(b)(6) may consider only the allegations contained in the pleading to determine its sufficiency."7 Santomenno ex rel. John Hancock Trust v. John Hancock Life Ins. Co. (U.S.A.), 768 F.3d 284, 290 (3d Cir. 2014) (emphasis added) (citing Pryor v. Nat'l Collegiate Athletic Ass'n, 288 F.3d 548, 560 (3d Cir.2002)). And while district courts are not required to accept merely conclusory factual allegations or legal assertions, see, e.g., Ashcroft v. Iqbal, 556 U.S. 662, 678-79, 129 S.Ct. 1937, 173 L.Ed.2d 868 (2009), they still must accept as true all plausible factual allegations made in the complaint and draw all reasonable inferences in the plaintiff's favor, see Bell Atl. Corp. v. Twombly, 550 U.S. 544, 555, 127 S.Ct. 1955, 167 L.Ed.2d 929 (2007); Williams v. BASF Catalysts LLC, 765 F.3d 306, 323 (3d Cir.2014). Accordingly, the District Court's reliance on facts alleged outside of Hassell's amended complaint constitutes a procedural error under Rule 12(b)(6).8

That said, the District Court's consideration of evidence extrinsic to the complaint does not automatically require reversal. Federal Rule of Civil Procedure 12(d) provides that "[i]f, on a motion under Rule 12(b)(6) or 12(c), matters outside the pleadings are presented to and not excluded by the court, the motion must be treated as one for summary judgment under Rule 56." See Messer v. V.I. Urban Renewal Bd., 623 F.2d 303, 307 (3d Cir.1980) ("[W]here matters outside the pleadings are considered by the district court, a motion under [Rule 12(b)(6)] for failure to state a claim upon which relief can be granted will be treated as a Rule 56 motion for summary judgment."); see also 5B Charles Alan Wright & Arthur R. Miller, Federal Practice & Procedure § 1366 (3d ed.2015).9 Although Rule 12(d) requires that the parties be given "reasonable notice," the failure to give notice may be excused as harmless error in the absence of prejudice to the nonmoving party. See, e.g., SBRMCOA, LLC v. Bayside Resort, Inc., 707 F.3d 267, 272-73 (3d Cir.2013) (quoting Ford Motor Co. v. Summit Motor Prods., Inc., 930 F.2d 277, 284-85 (3d Cir. 1991)).

In this case, the District Court did not notify the parties that it was converting Budd's motion to dismiss to a motion for summary judgment, but given that Budd alternatively presented its motion as one seeking summary judgment, it is unclear whether Hassell can demonstrate that she was unfairly prejudiced by the lack of notice. See In re Rockefeller Ctr. Props., Inc. Sec. Litig., 184 F.3d 280, 289 (3d Cir.1999) ("Failure to provide notice is harmless error if the plaintiff's complaint would not have survived a motion to dismiss."); Rose v. Bartle, 871 F.2d 331, 342 (3d Cir.1989) ("[T]he judgment may be affirmed if it appears that there is no set of facts on which plaintiffs could possibly recover."); see also Schering Corp. v. FDA, 51 F.3d 390, 400 (3d Cir.1995) (affirming summary judgment despite the district court's failure to give notice because the disputed issue — "the exclusivity of the statutory definition of bioequivalence" — was purely legal in nature). However we might consider the prejudice issue, vacating the order dismissing Hassell's complaint is still required because the District Court improperly applied the standard for summary judgment under Rule 56.

In a typical formulation of the summary judgment standard, it

Prowel v. Wise Bus. Forms, Inc., 579 F.3d 285, 286 (3d Cir.2009) (internal quotation marks and citations omitted) (quoting Norfolk S. Ry. Co. v. Basell USA Inc., 512 F.3d 86, 91 (3d Cir.2008)). The movant bears the burden of establishing the undisputed facts and entitlement to judgment as a matter of law. See El v. Se. Pa. Transp. Auth., 479 F.3d 232, 237 (3d Cir.2007) (quoting Celotex Corp. v. Catrett, 477 U.S. 317, 323, 106 S.Ct. 2548, 91 L.Ed.2d 265 (1986)).

Viewed through this lens, summary judgment was not appropriate in this case for at least three reasons. First, neither Budd nor Resco produced any evidence supporting their assertion that the railcar pipes responsible for Billie's asbestos exposure formed an "interconnected system" with the locomotive. App. 50a. Budd's first motion included exhibits in support of purely legal arguments concerning the scope of the LIA and the Federal Railroad Administration's regulatory authority, and its second motion was unaccompanied by any exhibits; Resco's joinder in these motions was similarly devoid of evidentiary support.10 Thus, the companies failed to carry their burden of proof on the purportedly converted motion. Second, even assuming that evidence for the "interconnected system" could have been gleaned from the record, Hassell attached in her opposition brief affidavit evidence from a former Railroad supervisor showing that, instead of being connected to locomotives, the pipes were connected to "power cars" that separately supplied steam heat to the passenger coaches. She therefore established a genuine dispute of material fact precluding summary judgment. See NAACP v. N. Hudson Reg'l Fire & Rescue, 665 F.3d 464, 475 (3d Cir.2011) ("After the movant shows that there is no genuine issue for trial, the non-moving party then bears the burden of identifying evidence that creates a genuine dispute regarding material facts."). Finally, the standard for summary judgment requires that factual inferences be drawn in the light most favorable to the nonmoving party. Although the facts before the District Court could have supported a reasonable inference that the pipes were connected to locomotives, summary judgment would have been improper because there were other facts of record supporting the contrary inference that the pipes were connected to something else. See, e.g., Anderson v. Liberty Lobby, Inc., 477 U.S. 242, 255, 106 S.Ct. 2505, 91 L.Ed.2d 202 (1986) ("Credibility determinations, the weighing of the evidence, and the drawing of legitimate inferences from the facts are jury functions, not those of a judge.... The evidence of the non-movant is to be believed, and all justifiable inferences are to be drawn in his favor."); see also, e.g., United States v. USX Corp., 68 F.3d 811, 827 (3d Cir.1995). Accordingly, whether viewed from the perspective of Rule 12(b)(6) or Rule 56, on the record as presented to us the District Court's order cannot be affirmed.11

* * *

For the foregoing reasons, we will vacate the District Court's order and remand Hassell's case for further proceedings consistent with this opinion.