Amtrak hasn't built much new rail. Amtrak maintains its rail and rolling stock and buys new rolling stock. Most of Amtrak's rail routes were originally made by the private rail companies that owned American rail during the Industrial era. Even the lines leased by freight rail was originally made by private rail companies. These companies were going bankrupt and the US government created Amtrak as a holding company to deal with selling off or winding down existing rail assets.