The rule usually given in formal writing is that if a compound subject joined by “or” or “nor” has subjects that differ in number, the verb takes the number of the noun closest to the inflected part of the verb.
Either the elephant or the lions are coming by train.
Either the lions or the elephant is coming by train.
That makes sense in terms of sound. After a plural noun, we expect a plural verb. After a singular noun, we expect a singular verb. It is not so much logic as it is euphony.
When subject and verb are inverted to form a question, euphony again suggests having the inflected part of the verb agree in number with noun closest to that part of the verb.
Is the elephant or the lions coming by train?
Are the lions or the elephant coming by train?
“Nor” is a different kettle of fish. Many style manuals insist on treating “nor” like “or.”
Neither Susan nor Jane is coming
does sound fine, but it makes no logical sense because it means
Susan and Jane are not coming
Personally, I do not find
Neither Susan nor Jane are coming
offensive, but some people will.