Non-EU: Travelling within the Schengen without a passport because its held at the US embassy

From talking and asking around, I know there have been many other people in my position and have helped me, so I'm passing it along here:

I was told I can email the US consulate to withdraw my passport so I can travel with it. I can then courier the passport back to them once I have returned from my travels. Since I am travelling immediately after the interview, they replied to my email (the next day) and said that I can ask to take my passport back right after the interview to travel. This way, I don't have to risk getting denied from boarding my flight at the airport.

Also, the other two answers I got here were right. I called up RyanAir & Eurowings and they were both very clear with me on the phone - you MUST have your passport. They also told me that the Residence Permit, though issued by Germany, which is a Schengen country, does not count as a travel document.

RyanAir, at least, has also stated on its website - non-EU citizens must get their travel documents (i.e. a passport) checked & stamped at the visa desk at the airport before going through security. They told me that they will not accept photocopies of passports. They have the right to not let you board your flight without your passport.

Eurowings doesn't talk about this anywhere on their website but the customer service agent said they sometimes do passport checks too. I know from my own past experiences of flying with Eurowings between Spain and Germany in 2018 and 2019 - I never had my passport checked and even when returning to Germany (Frankfurt), the border officer did not want to see my passport that I handed to him, but asked for my German residence permit instead. My guess is that my Schengen visa stamped in my passport had expired, so the residence permit card would be a better way of telling if I was allowed in or not.

As for train travel, I have a non-EU friend who travels almost every week by train on the London-Frankfurt and London-Paris route in the last 3 years, and has been asked to show her ID a couple of times by train conductors & police making their way through the train. She always had her passport on her - but she mostly always shows her German residence permit card, and they were always satisfied with that. It would still be a big risk, one that you shouldn't take if you can, especially if the police on board were to insist on seeing your passport - because apparently, that can happen.

This is just a clarification of some of the points in NYCvisitor2228968's essentially correct answer, but too long for a comment, so bear with me here.

Most, but not all, Schengen countries require you to carry valid and recognized travel documents when crossing their borders, also the borders between Schengen countries. Even if there is often no immigration checks when travelling between Schengen countries, you are still required to have your travel documents with you, just as you are required to have your driver's license with you when driving a car, although noone is checking your license everytime you are out driving.

I am not 100% sure about Austrian and Italian law, but Germany requires you to carry travel documents both when leaving and entering Germany, so no matter what route you take, any international travel plans will fail since you are not allowed to leave Germany.

Ryanair will definitely not let you fly, as they are very strictly verifying your travel documents and immigration status before they allow you to board. Most other no-frill airlines require you to show at least some kind of recognized id, which for most (if not all) non-EEA citizens is the same as a passport. Your residence permit is not valid as an id and it is not valid as a travel document.

Immigration checks at the border are quite common when travelling from Italy to Austria or from Austria to Germany. If you go by train, there is no id check to board the train, but you may be required to show an id to the conductor if you e.g. have a print-at-home ticket. Apart from that, you are even likely to reach Italy, but not unlikely to run into problems on your way back to Germany. Bus companies are on international lines also likely to check your passport when boarding and not let you travel without.

You need a valid travel document to cross borders, even within the Schengen area. A residence permit is not a travel document, so it is not enough. As documents may be checked at the gate, chances are you won't be allowed to board the flight.

Even if you take a train, there may be cursory checks on the border, especially if you go through the Brenner. You may encounter nothing going to Italy, but then have issues going back to Germany. Also, if you plan on staying in a hotel in Italy, you need valid ID, and a German residence permit won't be sufficient.