Deutshland-dicket terms of carriage § 2 (6) 5 says:
Zur Legitimation ist ein amtliches Lichtbilddokument mitzuführen und bei Kontrollen vorzuzeigen.
Amtlich means issued by a government agency.
Lichtbilddokument is a document containing a photograph.
- driver’s license (motor vehicle)
- electronic health insurance card (at least if issued by Germany’s statutory health insurance)
- firearms license
- mariner credential
- military ID
- national ID (including preliminary IDs)
- official ID card (e. g. of the police, customs, THW)
- passport (including diplomatic and emergency passports among others)
- radio operator’s license
- refugee travel document
- residence permit
- severely disabled person ID (at least if issued by the German Versorgungsamt)
- stateless person travel document
- student ID (from certain countries/universities)
- train driver’s license
Note, the terms of carriage do not say whether the presented document has to be still valid, neither is it required the government (agency) still exists (consider the GDR or FSU).
You see you have a rich trove of documents to choose from.
In reality, however, I am afraid showing your, say, Japanese Aeronautical Radio Operator license will not be accepted.
The ticket inspector will have a hard time to distinguish between genuine and counterfeit documents.
You will have the least troubles with a German national ID or a German passport.
That is for sure.
In border regions standard documents of the neighboring countries will be fairly well accepted, too.
Everything else is already risky.
You have a passport from Saint Lucia?
“Well, sorrrry, but I have never heard of ‘Saint Lucia’.
Sounds like a made-up country.”
Could a photocopy of the passport, or a picture of it in my phone be sufficient?
No, the terms of carriage indicate you have to carry the document on you.
A copy is a copy, not the document itself.
Given that I am not a German or EU citizen, what sort of ID should I carry with me? […] The only other photo IDs that I have are my own country's identity card and driving license […]
It is ridiculous, but carry them all.
Passport, ID, driver’s license, at times Germans can be a pain in the butt.
Are there any other options which I am missing?
No, except, of course, using a different ticket.
In some localities you will find, for instance, a regionally limited monthly pass ≤ €49 that is not tied to a specific person.
I presume your internship will not require traveling the whole nation anyways, so check your options.