Train Transfer Questions

Greetings!

I have a question regarding transferring trains/stations. I know that when I first board a train, I must show my JR Pass at either a manned ticket station or the small JR office at the station. But I am unsure of what to do when I have to transfer to a different train.

When transferring, can I go straight to the new train? Or do I have to show my JR Pass again at the new station? What if I am needing to transfer to the shinkansen?

Any help is greatly appreciated!

Thank you!!