I cannot answer all your questions but if you want to reserve a seat
in a train from cairo to alexandria

you can reserve online before even coming at
https://www.egypt.gov.eg/english/services/enr/default.asp

and I recommend that you start reserving within the next 2 days
because during this tiem of summer, train seats become sold out before
2 or 3 days of the train's departure

I also recommend that you get a 1st class seat at the Espanol train
(El Aspany) or the Turbo train (El Torbiny), because the're the ones
who are direct to alexandria without stops and have an air conditioned
first class seats,, otherwise, it may be annoying

the trip could take from 2-3 hours depending on the train and the
departure time, and there are trains to alex almost every 2 or hours
but the direct ones are about 3 a day, so check out good

if you want to reserve a train from egypt after you arrive, you don't
have to go to te train station itself in cairo (Mahattet Masr), you
can reserve a ticket from one of the 2 main phone centrals of Almaza
or Heliopolis (masr el gedida)

Hope that helps...

Ahmed Koraiem