Crossover bridges are a large three-part section of Cardassian Nor-class space stations such as Deep Space 9 that connect the Habitat ring to the docking ring.
Crossover bridges are composed of seventy percent rodinium, twenty-six percent kelindide and four percent toranium. (DS9 reference: Star Trek: Deep Space Nine Technical Manual)
Following the assassination of Shakaar Edon in 2376, Ro Laren ordered constant security patrols of the crossover bridges. (DS9 - Mission Gamma novel: Lesser Evil)