Least expensive/least crowded times for Amtrak travel

www.amtrak.com <-- they'll know better than we will. You can enter several different departure and arrival times and see differing fares. As for what's crowded, common sense dictates that it's times when people aren't traveling, e. g. no weekends, no holidays - most likely the least crowded times are either in the middle of the night or while most people are working. There are no guarantees re crowding, though. If a train is more or less empty, they can take cars off it.