Circumvesuviana Campania Express

You can buy tickets on the spot. No reason to pre-purchase ahead.
The Campania express is only one fancied up regular CV train that goes back and forth during non commute times. If it still runs next year, it's probably a good deal even at three times the regular price (about 15 euro instead of 4, peanuts anyway).
The issue however is that since when it was introduced in 2015, it has traveled more than half empty (actually about a dozen people, including the crew and two temps), therefore the Region of Campania had to subsidize it heavily so it turned out to be a huge failure with exorbitant costs. I guess not enough tourists willing to shell out the extra 10 euro per person. The Regional government may not be in the mood to subsidize tourists again this year for fear to incur into the Neapolitan tax payers' wrath, since they have to often endure below par government services in that area, so I'm not sure if they plan to operate the train again.

Thanks Roberto! Looks like we will just figure it out the day of.

Nothing particularly wrong with the old Circum.....

I'd agree, Frank. It's a battered and homely old thing but it gets the job done, and has more frequent runs than the Express...although that one has air conditioning so can be a nice plus in the heat of summer.