Amtrak announced plans Thursday to replace its fleet of high-speed trains on the East Coast. The railroad said that early next year, it would begin the process of replacing its 20 existing Acela Express train sets, which run on the Northeast Corridor rail line between Boston and Washington, DC. … Amtrak said Thursday that it had scrapped a previous plan to add 40 new passenger cars to the existing fleet, deeming it too expensive and “insufficient to handle new ridership growth projections.” …