We went to the station this morning; at the ticket window they told us to buy tickets on the train. But on the platform the police officers and railway staff informed us that we can not board the train. Foreigners are not allowed to ride on ordinary trains. Telling them we need to get a connection in Luxor didn't help.
My girlfriend speaks nearly perfect Egyptian Arabic, so communication was no problem. She had to take the case to the commander in charge of the station police and had to get very loud to get an exemption for us. We were then escorted to the train and seated in the special section reserved for police officers.
The ride was an interesting but loud and very smelly experience.
So at a major station is not possible to get on the train without speaking good Arabic and standing your ground.
At a small station without police you may be able to board and after the train is moving they won't kick you out at the next station.
They claimed something about missing insurance for the train class as the reason for not allowing foreigners.
I would not recommend the risk; try taking a deluxe train, it is not that much more expensive.