We demonstrate a concept for a large enhancement of the transverse magneto-optical Kerr effect (TMOKE) in the transmission mode. The concept exploits diffracted waves from dielectric-MO grating ribs to excite the guided modes in a high-refractive-index planar waveguide, thus opening different routes for efficient, integrated, and low-loss magneto-optical devices. The high $Q$ factors from very narrow peaks may permit applications in high-resolution sensing/biosensing.