USA Rail Passes are available in three travel durations and travel segments (15 days/8 segments, 30 days/12 segments and 45 days/18 segments). Amtrak considers a travel segment any time you get on and then get off a vehicle (train, bus, ferry or other allowable leg) regardless of length.
Because we don't have a direct route from Washington, DC to Oklahoma, City, OK, you could ride the Capitol Limited train from Washington to Chicago (segment 1), then ride the Texas Eagle train from Chicago to Fort Worth (segment 2) and finally ride the Heartland Flyer train from Fort Worth to Oklahoma, City (segment 3). This journey from Washington to Oklahoma City would use three of your allowable segments.
To travel from Spokane, WA to Portland OR, you could ride the Empire Builder train directly to Portland using one segment or you coulde ride the Empire Builder train to Seattle (segment 1) and then ride the Amtrak Cascades train from Seattle to Portland (segment 2). Both journeys take you to Portland but use a different number of your allowable segments.
Each segment booked counts toward the total allowable number of segments permitted for the pass.