The way you can normally tell in bike space is available is via the Amtrak advice:
Enter your origin and destination in the Fare Finder (for example, Los Angeles to Seattle). If bike service is available, an icon with a number of spaces available will display.
Select your desired journey option and proceed to the 'Add Ons' step to add bikes to your reservation.
I just did a test try - no biking icon appears for Cumberland to PGH or Washington Union Station to PGH.