The growing demand for increased flexibility and throughput capacity of automated warehouses is driving the implementation of rail-guided automated vehicle storage and retrieval systems (AVSRSs). A new type of AVSRS is capable of further increased performance by utilizing multiple vehicles along one rail instead of using only a single vehicle per aisle and tier. In this work, we introduce the shuttle vehicle scheduling problem (SVSP) in AVSRSs and present methods to solve the SVSP. We conduct a series of simulation experiments to show the performance improvement of horizontal transportation in various configurations of this new type of AVSRS and discuss main benefits of deploying high-powered AVSRSs.