Multi-stage fluidic mixing reactions are critical to diverse chemical and biological assays (e.g., immunoassays). Unfortunately, the majority of such assays suffer from laborious and time intensive fluidic mixing procedures. Although microfluidic platforms offer significant advantages for accomplishing biochemical assays, current systems primarily require external regulation during device operation, particularly for cases in which microbead visualization is desired or required during intermediate stages of reaction processes (e.g., aptamer-based sandwich assays). To overcome these limitations, here we present a microfluidic "rail-and-trap" processor that functions autonomously under continuous input flow conditions to both: (i) perform multi-step fluidic mixing routines with suspended microbeads, and (ii) immobilize select numbers of microbeads for visualization and/or optical detection during each step of multi-stage fluidic processes. Experimental results revealed railing efficiencies and trapping efficiencies of 100% for the prototype system, with microbead visualization achieved during each fluidic mixing stage.