They'll almost certainly use a custom unified cable to connect the headsets (probably just the cables all bundled together).
Looking at how the VR Coaster at universal works, it seems they use a Samsung Gear VR which is basically a galaxy phone. All the processing is done in the headset with no Computers on board the vehicles, just a wireless box.
If I had to guess how that solution works, it uses the 360 video tech like you see on youtube: https://www.youtube.com/watch?v=GFRPXRhBYOI
The videos are probably pre-downloaded and the wireless box just keeps the video in sync with the ride.
Not convinced they could get away with this on Ghost Train. The VR coaster takes a dumb terminal approach with minimal processing being done by the headset. You can't do interesting Augmented Reality effects and the VR would show everyone sitting in the same seats.
It's likely how it's being done for Air though.
An onboard computer to handle the number crunching? Being generous they could maybe do 5 headsets per server, that's a lot of rackspace. These servers will be really sensitive to vibrations and forces, any sort of drop would do major damage over a period of months-years. Even if the servers were kept static and the cars/seats moved independently, there will be significant vibrations that will be hard to dampen.
A bespoke wireless system that sends a compressed signal to the cars (with a smaller, more hardy piece of hardware to decompress and send to the appropriate headset on the train)?
Sending the data over powerlines on the track? You could run multiple lines with independent data streams, if the cars only ever travel at a low speed keeping contact for an unbroken stream should be simple enough. My home powerline network can do about 10megs a second which is comfortably enough for 10 high bandwidth video streams.