Okay, but the I-5 doesn't bother with loops and tunnels. It just barrels right over the top of the mountains, including a 5-mile section of 6% grade according to the sign (https://www.crashforensics.com/tejonpass.cfm). Trucks struggle to crawl over the top at like 30 mph, with tires. It's not exactly a roller coaster hill to be overcome with inertia.
They have distributed traction, meaning powered bogies in every second wagon for the current variants. There is no 'engine/locomotive' in the classical sense. All of these also have electronic anti-slip/traction control.
Edit: Even if a quarter of the powered bogies are defect for whichever reason, the remaining ones still suffice to accelerate from full stop at 4% grade. By design. For current variants. It's a variable platform. They could use more powered bogies. Stronger motors. Whatever.
