FAR 121.191
Cruising along at FL250 DEN-SLC over the Rockies and engine dies. Said Rocky Mountains are 14,000ish MSL. Our aircraft can only hold 13,000 MSL single-engine at today's weight. You must prove that you're not going to hit a mountain as you "driftdown" from your cruise altitude to your eventual level-off altitude. One has to clear terrain by 2000 AGL on the way down, and arrive over the landing airport 1500 AGL. The plan that is developed and included on our releases assumes that you'll turn around and go back to DEN if the failure occurs prior to the halfway point of the journey. Similarly, you'd continue to SLC if the failure occurs beyond the halfway point. If a failure was to occur in the middle portion of the route and in planning it was found you couldn't make either the destination or arrival cities without hitting a mountain, another city (or two or three) would be specified as your escape plan, a "driftdown alternate."

Alternatively the regs offer a less complex option: Maintain an "enroute net climb gradient" (i.e., maintain level flight or better) on one engine that keeps you 1000 above all terrain, period. This however, requires a lighter weight in the example above. We'd have to bump X number of people off until we could hold 15,000 on one engine.