The identification of the acoustic Green’s function (or, equivalently, the dynamic transfer function) of a medium is of interest to many fields, including structural testing, inspections and health monitoring. This paper will focus on the passive identification utilizing pairs of receivers and exploiting dynamic excitations that occur naturally in the structure. Several opportunities for this passive extraction exist, including: bridges under traffic excitation, buildings under seismic excitation, oceans under natural flows, and railroad tracks under train wheels excitations, among many others. A special signal processing approach is proposed to ensure that the Green’s function (or the transfer function) identification occurs without the influence of the random and generally unknown excitation and without the influence of uncorrelated noise that may affect the receivers. In particular, a special version of Welch’s periodogram technique is proposed where averages of the two outputs are taken both for the same time segments (“intra-segment” averaging) and for different time segments (“inter-segment” averaging) in order to eliminate the influence of noise at both receivers, in addition to eliminating the excitation source spectrum. It will be demonstrated, both analytically and experimentally, that this special signal processing is optimum for robust dual-output passive transfer function estimation. This technique will be then applied to the high-speed inspection of rail tracks by passive extraction of the rail acoustic Green’s function in the ultrasonic regime from the natural train wheel excitations. In this application, the dynamic outputs are collected by pairs of non-contact air-coupled receivers that have a 2-in stand-off from the rail surface. Changes in the passively-extracted Green’s function are then related to the presence of internal flaws (e.g. cracks) in the rail. Previously. a prototype based on this concept has been built and tested at the Transportation Technology Center (TTC) in Pueblo, Colorado, at speeds up to 80 mph. These speeds are unprecedented in the field of rail inspections, that are today carried out at ~30 mph at most by specialized test vehicles. This paper presents preliminary results from a second field test performed in the Fall of 2018 at TTC using a revised prototype design an speeds up to 40 mph. The successful development of this technique would revolutionize many aspects of rail maintenance by, for example, allowing regular trains to perform the inspections with no traffic disruption and great opportunity for redundancy due to the multiple train passes on the same track.