In the original plans for the Point Defiance railroad bypass south of Tacoma, Wash., something is missing: the sharp, 30 mile-an-hour curve where an Amtrak train derailed on Monday, killing three passengers.

The $11 billion Washington state government plan to speed up passenger and freight rail service throughout the Pacific Northwest called for the elimination of that tight turn—a change that wasn’t included in the final design when the state eventually won federal funding for the rail bypass.

Instead,...