Passenger trains carry people, while freight trains carry goods, raw resources, and even the mail. Because freight trains no longer need to carry passenger cars, engineers have changed their design. These changes mean that freight and passenger cars can no longer travel together.

Technically, yes, a freight locomotive can pull passenger cars because they use the same standard "knuckle" couplers and air brake systems; however, it is rarely done in regular service in 2026 for two reasons: Head-End Power (HEP) and Gearing. Freight locomotives typically lack the secondary generator or alternator needed to provide electricity (HEP) for the passenger cars' air conditioning, lighting, and Wi-Fi. Without a specialized "power car," the passengers would be in the dark and without climate control. Furthermore, freight engines are geared for torque and heavy loads at lower speeds (usually capped at 60–70 mph), whereas passenger locomotives are geared for higher speeds (up to 110–125 mph). While you might see a freight engine "rescuing" a broken-down Amtrak train at low speed, using them for daily passenger service would be highly inefficient and significantly slower than the dedicated passenger units used across the US and Europe.