The barge dock on Harbor Island is an interesting arrangement. I've seen other RR docks where the vessel has switches onboard to allow access to a greater number of tracks than the ramp has.

This particular ramp has two tracks, but there are 6+ tracks on the barges I have seen. Once the initial two tracks are worked, the barge is shifted sideways to access a different set of tracks. This shifting of the barge is repeated until the entire vessel is worked.

There are 4 large pylons in the water outside the corners of the barge loading area with cables that attach to the corners of the barge. The winches on the other end of those cables are activated to shift the barge sideways when repositioning to other tracks is needed.

The barge slip is visible in the satellite view in Google Maps:
[www.google.com]
Currently, you can see a barge in place at the slip with moderate levels of map enlargement. If my link worked as I intended, there is a barge in place when you click the link. Look for the colorful visible containers stacked cross-wise on the barge to identify the barge. If you zoom in more for greater detail, the barge disappears. Additionally, note an older disused ramp to the right of the active ramp.

Also, as you can see in Steve's photos, the barge has containers stacked on racks above the railcars. I believe those containers are loaded/unloaded at a different Seattle area terminal, but I don't know exactly where that occurs.