Are there hop on hop off buses at Austerlitz Train Station? If so, how long is the tour?
The Paris l'Opentour has four routes. Buses on the blue route stop at Gare d'Austerlitz. See this brochure.
Thank you - can you change and get on a green route if so desired? The problem is we get in at 12:30, need to transfer from Nord to Austerlitz, put bags in lockers, and then catch a night train at 7:30. Will we even have time to do the whole green route?
Yes, the brochure shows a transfer point from the blue route to the green route on l'Ile de la Cite by Notre-Dame. Central Paris is quite compact. If you don't get off the bus except to transfer, I think you would easily be able to complete the green route. But I'm only guessing.