A bit confusing as to Kelso --e.g., maybe you can make a reservation for your bike (from the Amtrak site):
BICYCLES
Make a reservation for both you and your bike aboard Amtrak Cascades. There are two options for bringing your bike – you can use one of our onboard bike racks, or box your bicycle and check it as baggage.
Ten bike racks are available on each Amtrak Cascades train. Bike racks are very popular and reservations are required. The cost for reserving space in the bike rack is $5. Bicycle racks will not accommodate tandem, recumbent or other unusual bikes. Such bikes must be transported as checked baggage in a box.
Boxed bicycles do not require a reservation and can be checked as baggage. There is a $10 handling fee for checking a boxed bike. (This is subject to change without notice.) Not all stations have checked baggage service, and baggage service may not be available every day. Don't have a bicycle box? Bicycle boxes are available for purchase for $15 at staffed Amtrak stations with checked baggage service. Bicycles usually must be partially disassembled. For details regarding boxing your bicycle, on general information bringing bicycles onboard, please visit
Amtrak.com
Amtrak Cascades - Baggage