It's a start.
Amtrak's booking engine (Amtrak.com) now lists the percentage of seats that have been sold for each departure. That means that before purchasing a ticket in coach class, customers can suss out just how easy—or difficult—it will be to practice social distancing aboard any given train.
An example: In the screen shot above, which lists journeys from St. Louis to Chicago, two of the departures are 80% full. The last departure of the day, however, is far more lightly sold at just 20%. So a passenger who wants extra space would book that late train, knowing it won't be packed with fellow passengers and that carriages will be only lightly occupied. (Amtrak's coach cars use general seating, not seat assignments).
To be certain that a given train won't fill up too much after you buy your ticket, making a day-of booking will usually be smarter than an advance sale.
To be 100% certain you'll have a row to yourself on the train, you could spend a bit more and buy a business class seat or opt for a route that uses reserved seating, such as Acela. According to Jason Abrams, Amtrak's Public Relations Manager, "For trains with reserved seating, only window seats and the front-facing table seats are being assigned. The rear-facing table seats and the aisle seats have been removed from the seat maps."
Bottom line: In business class and on the few routes that use reserved seating, Amtrak intentionally leaves seats vacant so that passengers don't sit beside anyone.
For more on what Amtrak is doing to prevent viral exposure, the carrier published an announcement about its cleaning procedures and another about its new safety partnership with George Washington University.