Structures for a waveguide crossing and methods of fabricating a structure for a waveguide crossing. A waveguide crossing includes a central section and an arm positioned between a waveguide core and the central section. The arm and the waveguide core are aligned along a longitudinal axis. The arm is coupled to the waveguide core at a first interface, and the arm is coupled to a portion of the central section at a second interface. The arm has a first width at the first interface, a second width at the second interface, and a third width between the first interface and the second interface. The third width is greater than either the first width or the second width.