In this appeal, we must decide whether the Railway Labor Act (“RLA”) bars Burlington Northern Railroad Company (“BN”) from unilaterally implementing a mandatory urine testing program designed to restrict alcohol and narcotics use by on-duty railroad workers. The program requires the entire operating crew of a train to submit to urinalysis when the train is involved in a “human factor” accident unless “responsibility [for the accident] is clearly identified.” When responsibility it clearly identified, only those responsible for the accident are tested.
Our resolution of this matter hinges on whether the challenge of the Brotherhood of Locomotive Engineers (“BLE”) to BN’s urine testing program is a major or minor dispute under the RLA. The district court held that it is a minor dispute because it is “arguably justified” under an implied provision of the collective agreement between BN and BLE. We disagree.
FACTS
For at least 40 years, BN, like many railroads, has enforced a unilaterally implemented safety rule, Rule G. Rule G prohibits BN employees from using alcohol or narcotics while on duty, from possessing such substances while on company property, and from reporting for duty in a state of intoxication that may impede their ability to perform their work safely.1 The collective bargaining agreements between BN and its employees’ unions do not expressly or implicitly refer to Rule G.
*1089Undisputed evidence presented to the district court indicated that, before May 1984, BN relied primarily upon sensory surveillance to enforce Rule G. If an employee’s gait, breath, odor, slurred speech, bloodshot eyes, or errant behavior suggested to a supervisor or security officer that the employee was intoxicated, BN suspended the employee pending a formal investigation under established procedures. An employee could avoid suspension by voluntarily submitting to urinalysis.
On April 15, 1984, a train crash in Wiggins, Colorado killed five BN employees and caused $2 million in property damage. The National Transportation Safety Board implicated alcohol abuse by an engineer as a possible cause of the accident. On April 21, a train disobeyed an “absolute stop” signal and crashed into a standing train at Newcastle, Wyoming. Two BN employees died. Property damage totaled $1 million. A toxicology report indicated that three crew members had marijuana traces in their body fluids.
Following these two serious crashes, BN intensified its efforts to enforce Rule G by introducing two new programs. First, BN contracted for trained sniffer dogs to detect narcotics on its premises.2 Second, it began requiring all crew members involved in a human factor accident or operating rule violation to attend a clinic immediately and submit to urinalysis for the presence of narcotics, unless responsibility for the accident or violation otherwise has been clearly identified. Under the new rule, an employee who refuses to submit to mandatory urinalysis after being involved in a human factor accident or operating rule violation is subject to discipline for insubordination.
BLE strenuously opposed BN’s new urine testing program and urged BN to end it. BN refused to negotiate. BLE then balloted its branch chairmen on whether to strike on the urinalysis issue. The majority of respondents voted in favor of a strike. BLE filed suit under the RLA. The district court found that BN’s urine testing program was “arguably justified” under the collective agreement as amended by custom and practice, and thus constituted a "minor dispute” under the RLA. Brotherhood of Locomotive Eng’rs v. Burlington N.R.R., 620 F.Supp. 173, 175 (D.Mont.1985). Accordingly, the court concluded that it lacked jurisdiction over the dispute, and granted summary judgment in favor of BN. BLE appeals. We reverse.
STANDARD OF REVIEW
Whether a matter constitutes a mandatory subject of bargaining under the RLA is a question of law which we review de novo. See Japan Air Lines Co. v. IAM, 538 F.2d 46, 52-53 (2d Cir.1976); cf. NLRB v. Int’l Harvester Co., 618 F.2d 85, 87 (9th Cir.1980).
Whether past conduct constitutes custom and practice sufficient to be incorporated as an implied term in a collective agreement covered by the RLA is a question of fact. See Missouri Pac. Joint Protective Bd., Bhd. Railway Carmen of the United States and Canada v. Missouri Pac. R.R., 730 F.2d 533, 537 (8th Cir.1984); cf. Hass v. Darigold Dairy Prods. Co., 751 F.2d 1096, 1101 (9th Cir.1985) (implied terms in collective agreement under NLRA). We accept a district court’s determination of questions of fact unless clearly erroneous. United States v. McConney, 728 F.2d 1195, 1201 (9th Cir.) (en banc), cert. denied, 469 U.S. 824, 105 S.Ct. 101, 83 L.Ed.2d 46 (1984).
Whether a dispute constitutes a major dispute or a minor dispute under the RLA is a matter of law which we review de novo. IAM v. Aloha Airlines, Inc., 776 F.2d 812, 815 (9th Cir.1985).
ANALYSIS
BN presents alternative arguments in support of its contention that the district court may not enjoin the mandatory urine testing program. First, BN contends that the enforcement of Rule G is a matter *1090entirely within its managerial prerogative. Alternatively, BN contends that the challenged procedure presents only a minor dispute — over which the district court lacks jurisdiction — because it is arguably justified by implied-in-fact and implied-in-law terms of the collective agreement. We address these arguments in turn.
I. Management Prerogative
Neither party disputes that Rule G itself is an implied term of the collective agreement. BN asserts, however, that selection of the method used to enforce a safety rule like Rule G is entirely a matter of management prerogative, not subject to collective bargaining under the RLA. Thus, BN contends, its prior enforcement practice cannot be an implied condition of the collective agreement or give rise to a labor dispute within the terms of the RLA.
Under the RLA, certain matters are always subject to collective bargaining; other matters fall within the scope of management prerogative, and are not subject to collective bargaining. The RLA requires railroads “to exert every reasonable effort to make and maintain agreements concerning rates of pay, rules, and working conditions.” 45 U.S.C. § 152 (emphasis added). No court has comprehensively explained the scope of the statutory phrase “working conditions.”
In Japan Air Lines Co. v. IAM, 538 F.2d 46, 52 (2d Cir.1976), the second circuit upheld an employer’s management prerogative to determine whether to cease subcontracting ground work at certain airports because the “primary impact” of the union’s proposal did not affect the working conditions of the union’s present members. 538 F.2d at 52. In so holding, the court distinguished the facts of the case before it from those presented in Fibreboard Paper Products Corp. v. NLRB, 379 U.S. 203, 85 S.Ct. 398, 13 L.Ed.2d 233 (1964), and Order of Railroad Telegraphers v. Chicago & North Western Railway Co., 362 U.S. 330, 80 S.Ct. 761, 4 L.Ed.2d 774 (1960), in which “the critical question was whether those who were working were to lose their jobs.” 538 F.2d at 52-53.
Thus, the Japan Air Lines court set forth a simple and pragmatic approach: The RLA requires parties to bargain over any proposal whose primary impact is the loss — or potential loss — of existing employment or employment-related benefits. We now adopt this approach as the rule of this circuit. Under BN’s chemical testing program, a Rule G violation, however discovered, could result in an employee’s discharge. Because jobs of present employees are jeopardized, BN’s mandatory urine testing program is a mandatory subject of bargaining under the RLA and may not be implemented unilaterally. Thus, BN’s management prerogative claim is unfounded.
II. Major vs. Minor Dispute
Under the Railway Labor Act, all disputes between railroads and their employees are either “major disputes” or “minor disputes.” These two categories are “sharply distinguished.” See Elgin, Joliet & Eastern Ry. v. Burley, 325 U.S. 711, 722, 65 S.Ct. 1282, 1289, 89 L.Ed. 1886 (1945), aff'd on rehearing, 327 U.S. 661, 66 S.Ct. 721, 90 L.Ed. 928 (1946); O’Donnell v. Wien Air Alaska, Inc., 551 F.2d 1141, 1145-46 (9th Cir.1977) (tracing historical development of RLA).3
Major disputes “concern [either] the formation of collective bargaining agreements or efforts to secure new rights and incorporate them into future agreements.” Aloha, 776 F.2d at 815 (citing Elgin, 325 U.S. at 723, 65 S.Ct. at 1290). Minor disputes involve the interpretation or application of existing collective bargaining agreements. Elgin, 325 U.S. at 723, 65 S.Ct. at 1290; Aloha, 776 F.2d at 815.
Generally, major disputes are “left for settlement entirely to the processes of non-compulsory adjustment.” Elgin, 325 U.S. at 723-24, 65 S.Ct. at 1290. A detailed procedure is provided in 45 U.S.C. §§ 152, 155, 156, 157, and 160 for moving the dis*1091pute through initial conference and negotiation, submission to the National Mediation Board, voluntary arbitration, and, possibly, investigation by a special emergency board appointed by the President. See Brotherhood of R.R. Trainmen v. Jacksonville Terminal Co., 394 U.S. 369, 378, 89 S.Ct. 1109, 1115, 22 L.Ed.2d 344 (1969). “No authority is empowered to decide the dispute and no such power is intended, unless the parties themselves agree to arbitration.” Elgin, 325 U.S. at 725, 65 S.Ct. at 1291. Pending the exhaustion of the RLA’s mechanisms for resolving a major dispute, the status quo applies. The duty to maintain the status quo may be enforced by obtaining injunctive relief in a federal district court. See Aloha, 776 F.2d at 816.
In contrast, minor disputes are subject to compulsory and binding arbitration. See 45 Ü.S.C. § 153(first)(i). This task is performed by the National Railroad Adjustment Board or a privately established arbitration panel. See generally Seidenberg, Grievance Adjustment in the Railroad Industry in The Railway Labor Act at Fifty 229-35 (National Mediation Board ed., 1976). The arbitrators have exclusive jurisdiction over minor disputes. See Slocum v. Delaware, Lackawanna & W.R.R., 339 U.S. 239, 244-45, 70 S.Ct. 577, 580, 94 L.Ed. 795 (1950). Thus, federal courts have no subject matter jurisdiction over minor disputes. Aloha, 776 F.2d at 815. When only a minor dispute is involved, there is no duty to maintain the status quo and judicial review of the arbitration panel’s decision is extremely limited. See Union Pac. R.R. v. Sheehan, 439 U.S. 89, 91, 99 S.Ct. 399, 401, 58 L.Ed.2d 354 (1978) (per curiam).
The test in our circuit for determining whether a dispute involves only the interpretation or application of an existing agreement (and is therefore minor) or involves the formation of a collective agreement or a unilateral effort to change working conditions (and is therefore major) was established in Switchmen’s Union of North America v. Southern Pacific Co., 398 F.2d 443, 447 (9th Cir.1968). See Aloha, 776 F.2d at 815-16; O’Donnell, 551 F.2d at 1146-47; Southern Pac. Transp. Co. v. United Transp. Union, 491 F.2d 830, 832 (9th Cir.), cert. denied, 416 U.S. 985, 94 S.Ct. 2389, 40 L.Ed.2d 762 (1974). We have reformulated the test several times. See Switchmen’s, 398 F.2d at 447 (only if the disputed action is “in nowise contemplated or arguably covered by the agreement” does it give rise to a major dispute); Southern Pac. Transp. Co., 491 F.2d at 832-33 (A dispute is minor when a contract provision is “reasonably] susceptible” to a party’s interpretation.); O’Donnell, 551 F.2d at 1146 (“Is the position of at least one of the parties arguably predicated on the terms of an agreement?”); Aloha, 776 F.2d at 816 (major dispute concerns issue “neither contemplated nor arguably covered” by collective agreement).
The district court accurately summarized the essence of these various reformulations as whether the railroad’s actions are “arguably justified” by the collective agreement between BN and the BLE. See 620 F.Supp. at 175. If the railroad’s actions are arguably justified, the dispute is a minor dispute; if they are not arguably justified, the dispute is a major dispute. When in doubt, courts construe disputes as minor. See O’Donnell, 551 F.2d at 1146-47 (Switchmen’s test “is not a stringent one”); Brotherhood of Locomotive Engineers v. Atchison, Topeka and Santa Fe Railway Co., 768 F.2d 914, 920 (7th Cir.1985) (citing cases).
A railroad collective agreement includes not only express terms, but terms implied by past practice and by law. See Detroit & Toledo Shore Line R.R. v. United Transp. Union, 396 U.S. 142, 153, 90 S.Ct. 294, 301, 24 L.Ed.2d 325 (1969) (the status quo in a major dispute includes employment terms established by past practice); Maine Cent. R.R. v. United Transp. Union, 787 F.2d 780, 782-83 (1st Cir.) (railroad’s assertion that proposed action comported with past custom and practice was arguably correct and thus dispute was minor), cert. denied, - U.S.-, 107 S.Ct. 169, 93 L.Ed.2d 107 (1986); Railway Labor Executives’ Ass’n v. Atchison, Topeka & Santa Fe Ry., 430 F.2d 994, 996 (9th Cir.1970) (parties’ failure to include a provision in the agreement *1092may indicate that they were relying on the language of a statute to fill the gap), cert. denied, 400 U.S. 1021, 91 S.Ct. 582, 27 L.Ed.2d 632 (1971).
It is clear that BN’s new mandatory urine testing program does not fall within the scope of any express provision of the collective agreement. Therefore, in determining whether BN’s chemical testing program constitutes a major or a minor dispute, we must decide whether it is “arguably justified” by any implied provision of the agreement. If it is “arguably justified” by either a term implied by past practice (an “implied-in-fact” term) or a term implied by statute or regulation (an “implied-in-law” term), then the dispute is minor, and the federal courts lack jurisdiction to enjoin the program.
A. Implied-in-Fact Terms
As an initial matter, we note that both Rule G and its customary enforcement by sensory surveillance were implied-in-fact terms of the collective agreement between BN and BLE. Several BLE officials with extensive experience regarding BN’s operating procedures testified that sensory surveillance by BN’s supervisors had been the only regular method of Rule G enforcement for over forty years. BLE had long acquiesced in this practice. BN has never disputed that sensory surveillance by BN’s supervisors was the primary means of detecting Rule G violations before 1984.
BN contends that its urine testing program is arguably justified by the past practice of sensory surveillance. In the companion “sniffer dog case,” the district court determined that the custom and practice of relying on sensory surveillance to detect Rule G violations arguably justified any reasonable enforcement method based on “a modicum of evidence.” Brotherhood of Locomotive Eng’rs v. Burlington N.R.R., 620 F.Supp. 163, 171 (D.Mont.1985). In the instant case, the district court concluded that such an implied condition arguably justified BN’s mandatory urine testing program because “the use of urinalysis was [not] done on a completely random basis. Rather.... BN subjects an employee to urinalysis when the employee is believed to have been involved in an operating rule violation.” Brotherhood of Locomotive Eng’rs v. Burlington N.R.R., 620 F.Supp. 173, 175 (D.Mont.1985). This conclusion ignores the critical differences between the old method and the new method: The old method of enforcing Rule G was voluntary, and required particularized suspicion; the new method is mandatory, and requires only generalized suspicion.
We do not agree that BLE, by acquiescing in Rule G’s enforcement by sensory surveillance, can be said to have agreed to allow BN to implement any procedure beyond sensory surveillance so long as the procedure is brought into play by a “modicum of evidence” or an “objective triggering event.” The kind of sensory surveillance that BN’s supervisors had employed in the past — observing an employee’s gait, breath, odor, slurred speech, or blood shot eyes — was non-intrusive. It did not implicate the serious privacy intrusions posed by BN’s use of mandatory urine testing. Moreover, the sensory surveillance employed by BN’s supervisors in the past required suspicion of individual workers based on their conduct and appearance, whereas the new mandatory urine testing procedure may be imposed on the basis of generalized suspicion encompassing an entire train crew.
Our analysis here is aided by reference to fourth amendment doctrine. BN is not a government agency and, therefore, is not subject to the restrictions of the fourth amendment. However, the focus of our inquiry — both under the fourth amendment and under an implied provision of a collective agreement — is the expectation of privacy of those who will be subject to urine testing. See O’Connor v. Ortega, — U.S. -, 107 S.Ct. 1492, 1497-99, 94 L.Ed.2d 714 (1987) (reasonableness of a search under the fourth amendment depends on reasonable expectations of privacy); Shoreline, 396 U.S. at 153, 90 S.Ct. at 301 (the terms of a collective agreement include “actual, objective working conditions and practices, broadly conceived. . . .”).
*1093In Railway Labor Executives’ Association v. Burnley, 839 F.2d 575 (9th Cir.1988), we struck down federal railway safety regulations requiring a mandatory chemical testing program similar to BN’s program at issue here. We concluded in Burnley that the fourth amendment requires “a degree of particularized suspicion” of the use of alcohol or a controlled substance before a railroad employee can be subject to mandatory drug testing. This analysis is readily applicable here. A railroad worker has certain expectations regarding whether the government may invade his or her privacy under the fourth amendment. Similarly, a railroad worker has certain expectations regarding whether an employer may engage in a similar invasion of privacy under an implied provision of a collective agreement. These expectations are related: Although the source of the invasion is different, the privacy interest is the same. In the case of urine testing programs, the Constitution draws a line between those programs based on particularized suspicion and those based on generalized suspicion. Burnley at 584. The former are permissible; the latter are not. We decline to assume that BLE members implicitly granted BN the authority to invade their privacy in ways the government could not.
BN’s new mandatory urine testing program is a clear change in working conditions governed by the collective agreement and, thus, by definition, a major dispute. It is a mistake to conclude that BLE impliedly agreed to the new testing program just because “through custom and practice” it had for forty years accepted enforcement of Rule G by the non-intrusive procedure of sensory surveillance. BN’s new procedure requiring mandatory urine testing of all members of a train’s crew is not arguably justified by the implied provision in the collective agreement allowing BN to enforce Rule G by voluntary urinalysis based on information from sensory surveillance.
B. Implied-dn-Law Terms
BN also contends that its new mandatory testing program is justified by an implied-in-law provision of the collective agreement. It argues that federal regulations mandating chemical testing of railroad workers constitute an implied-in-law term of its agreement with BLE.
It is true that Federal Railway Administration regulations mandated a chemical testing program similar to the program imposed unilaterally by BN. See 49 C.F.R. §§ 219.201, 219.203, and 219.213. However, we have struck these regulations down on constitutional grounds. Railway Labor Executives Ass’n v. Burnley, 839 F.2d 575. They no longer retain any legal force. No other legislative or administrative pronouncement stands either for or against the challenged program. Therefore, generalized mandatory urine testing is not an implied-in-law condition of the collective agreement, and BLE’s resistance to that program constitutes not a minor, but a major dispute.
REVERSED AND REMANDED.
. Rule G presently states:
The use of alcoholic beverages, intoxicants, narcotics, marijuana, or other controlled substances by employees subject to duty, or their possession or use while on duty or on company property is prohibited. Employees must not report for duty under the influence of any marijuana, or other controlled substances, or medication, including those prescribed by a doctor, that may in any way adversely affect their alertness, coordination, reaction, response or safety.
BN unilaterally added the reference to “marijuana, or other controlled substances” in 1980.
. BLE’s challenge to BN’s sniffer dog program is addressed in our companion case, Brotherhood of Locomotive Engineers v. Burlington Northern Railroad Co., 838 F.2d 1102 ("the sniffer dog case”).
. The terms "major dispute" and "minor dispute” are not found anywhere in the RLA. The Supreme Court in Elgin coined the terms as a methodology to be used by courts confronted with disputes arising under the RLA. See Aloha, 776 F.2d at 815 n. 2.