Just go to the station - no more than the day before - probably better to go the same day and purchase your ticket. I don't know the cost, but it must be under 50LE as that is what the first class train costs. It's probably closer to 30LE. Seats are assigned.
One of the bus stations is across the street from Sidi Gaber train station.
Another idea for a cheap, but probably life threatening way to get to Cairo would be a micro bus - you can go down to the main train station in Alex and there are micro buses there by the main gate (at least at night there are) heading to Cairo trying to fill up. They are probably cheaper, but very scary.

I really suggest spend the 50LE and take the train. It's significantly safer, and when you hit Cairo you know where you are - down town at Ramses Station. You've got family to transport, don't you? The train is also significantly more comfortable. You can take 2nd class seats on the fast train for less money, or go on one of the trains that takes a bit more time and has a stop or two - they are less expensive too. You should get those tickets up to 2 weeks in advance if possible as the trains are very popular in the summer.... as will the Super Jet I suppose if you are traveling at a peak hour on the weekends.

Whats a super jet bus? interesting!

Thanks so much Casual_Cairo!

Miss Green - Super Jet is just one of the names of the bus companies here - like GreyHound in the USA - don't know what your bus companies are called in Oz, but sure there is something similar.

CC... We have Greyhound.. but nothing as fabulous sounding as Super Jet!

LOL - The only thing "fabulous" about Super Jet (if that) is the name. The buses are nothing to write home about. They are OK, and there is a hostess (sort of) on board that will serve you meals, drinks, etc. for an expensive fee if you need that. That was the one thing that was unusual about it. Also, of course there are movies playing. Does GreyHound do that now?

How far is train status from Alexandria cruise pier? Any idea how much a taxi would cost for this travel. Thanks.

The taxis at the port gate are looking for longer fares so they will not be cheap to get to the train station. You might have to pay as much as $10 for what might normally be a $3 or $3.50 fare. If you will cross the street at the port and walk about 3 blocks in toward town, you might get a taxi for less.

Thanks Casual Cairo. How much time I would need to go from cruise terminal to train station. My ship arrival is 6 am. Can I get 8 am train for Cairo?

You SHOULD be able to get on the 8 a.m. train, but of course there is no guarantee. You could hit bad traffic (shouldn't at 6 a.m.) or get a really stupid taxi driver, or anything, but most often it can happen. Just be ready at the train station for issues with language. Everything is NOT going to be marked in English, so be ready for charades nd what ever else it takes to get your tickets providing tickets for the train you want are still available. If first class is sold out, ask about 2nd class as it will arrive at the same time (pulled by the same engine) and 2nd is not bad. Same train, just narrower seats.

Have fun.