Let me echo what has already been said. Be prepared for ANY type of weather possible. This means everything from 35 degrees, a 15mph wind and heavy rain to 70 degrees and sunny. The weather in Cass itself has no bearing whatsoever as to what it's doing on the mountain. That 35 degrees thing was the mountain. Cass, for the entire day was about 60 and partly cloudy. People were wearing gloves and putting their hands on the fireboxes to keep warm. The next day was the 70 degrees and sunny. Bring clothes for all weather conditions. Spring, Summer, Fall, Winter, rain, sun, snow, and dress in layers. Trust me, you will be happy you did. I've almost frozen to death at Cass more than any other place because what you THINK you need for the day, and what you end up with on the mountain can be total opposites. It's easy to shed clothes. Not so easy to put them on when you don't have them.
Next point. Do not expect to find many "modern" conveniences nearby. For gas, you will have to drive either to Snowshoe or Marlinton, and neither are cheap. We always top off on I-64 on the way in. It's not really cheap there either, but it's cheaper than anything else you will find. Speaking thereof, coming from central NC, we always got off at White Sulphur Springs and came in on 92/39. After various attempts, I always thought it was a bit easier drive as opposed to 219. That said, there is no easy way to get there from anywhere. It's in the middle of nowhere. Oh, don't expect any cell phone reception nearby. The government has some big satellites in Green Bank (north of Cass), and they block out cell reception for the surrounding area. They also have free tours, which are pretty neat.
Next....be ready for lots of possible fog and LOTS of wildlife. It's deer country and they are everywhere, including the roads. If you are staying in Snowshoe, give yourself plenty of time to get there. Driving in broad daylight is one thing. Driving in the dark with deer is something else.
As far as runbys are concerned, Joe Gonzalez and the crew will have those fairly well marked off and organized. There are benefits to being on either the far end of the photo line, or in the curve of the photo line (if applicable) so as to keep people out of the shot. When you are dealing with that many people, you have to have organization (and control) or things deteriorate rapidly. Joe's good about that kind of thing. Rule #1...if you are out of the line and can see someone, they can see you, and will not be too happy about it, then you get Joe upset with you. Best to not foul anyone's shot! He's a good guy though--don't mistake this.
Now, as far as other stuff goes, allow yourself some time to explore the town. There are some nice model railroads there, and the town itself is cool. Some of the coolest moments are being there on Friday afternoon, or in the early mornings when they are getting stuff ready. If there is a shop tour, that's pretty cool also.
I'm not exactly sure how they are running Railfan Weekend these days (it's been a while since we did that and always came up for other things), but there is a strong possibility that they will run the consist as one long train leaving Cass. That means the back open air car is the best seat in the house with a Shay right behind you. With the number of people, they may run the trains separate up the mountain, but the above-mentioned scenario happens a lot with "smaller" groups.
If you have time, run up to Durbin to see the Climax. About a 30 minute drive, and the feel between Cass and Durbin are total opposites. If you want to know what a logging railroad in WV was like, ride that line. Cass, being a state park for 50 years, is maintained to a very high standard to the point that much of the "logging railroad" feel has been lost. The Durbin line was washed out several times, and operates on somewhat less of a budget, and some interesting trackwork. Just being there in town and watching things come to life can be pure magic and a real time warp to the 1950s. Speaking of magic, there's a little bar/restaurant about 2 doors up (ie heading back towards Cass) from the gift store/ticket agent in Durbin that has some of the best hamburgers I've ever had. Well worth the time, and as you will have no doubt figured out, there isn't much to eat anywhere near there.
Other than that, be safe and have fun. Cass is a very cool place.
Kevin