Based on statistics published at the end of February 2016, the Technical Report Archive & Image Library (TRAIL) has deposited over 39,000 volumes in the HathiTrust! This places TRAIL in the top 25 of HathiTrust content contributors which are mostly universities, with the notable exceptions being the Library of Congress, The Getty Research Institute, and TRAIL. A list of content contributors can be found here.
Deposited volume counts by original source of content are published monthly by HathiTrust and TRAIL has contributed 39,806 volumes to date. This in addition to the now more than 18,000 volumes on deposit in the University of North Texas (UNT) Digital Library!
The TRAIL Search Interface can be used to search both these repositories simultaneously for TRAIL content.