Defense Transportation: The Army Should Take Action to Better Ensure Adequate Rail Support to Combatant Commanders, Report to Congressional Committees [open pdf - 794KB]
From the Highlights: "According to Army officials and doctrine, rail is the most cost effective and expeditious means of moving large quantities of materiel, such as tanks and ammunition, over long distances. Army officials also stated that during contingencies, approximately 67 percent of Army unit equipment moves by rail from its fort or base of origin to a shipping port. House Report 116-120, accompanying a bill for the National Defense Authorization Act for Fiscal Year 2020, included a provision for GAO [Government Accountability Office] to review military rail capabilities. GAO evaluated the extent to which the Army has taken action to 1) meet potential needs for rail operating crews and 2) inspect, repair, and monitor installations' rail track."
Government Accountability Office: https://www.gao.gov/