This issue includes 25 papers that were presented at the First International Symposium of Advanced Train Control, held in Denver, Colorado, in June 1991. The principal purpose of this symposium was to bring together a broad cross section of railroad and supply company officials to demonstrate the unique application of advanced train control to all aspects of railroading, to share ideas and technology, and to enumerate the benefits of these systems. Advanced train control means the use of microprocessors and digital data communications to provide new command, control, communication, and information tools to improve railroad operations. The program structure was designed to present the business case for advanced train control, followed by technical sessions that describe the design and implementation of these systems. Papers from the technical sessions are published in this issue, along with papers from a session on advanced train control in Europe.