I'm no expert, but I imagine the most cost effective way to do this would be to have wifi along the lines rather than on the trains themselves - even if I'm wrong about that, installing wifi onto a train isn't something that needs to be built into the train from conception, and is something that anything they plan into it now will probably be obsolete in its tech standards by the time these trains launch in ~10 years.

On the subject of current station wifi - who has to pay? Certainly for me (Vodafone customer) I get it free, it's still provided by Virgin but they give a semi-whitelabelled login page for Vodafone (I don't have to login, it just takes 10 seconds when it opens the authentication page and verifies me on the network). I also find it great (albeit not as great as solid coverage all along the line) to be able to hook back onto the network as I travel through stations. It doesn't require me to re-auth each time, so essentially every minute or two my phone can sync emails, twitter, whatsapp, etc. leaving me the actual driving time between stations for reading/responding as needed.

2G/3G/XG is kinda becoming irrelevant and is provider-dependant.

They should have WiFi hotspots in each train so you wouldn't be reconnecting every 5 seconds as the train moves. Also it's probably easier to connect train to whatever central network pipe.

I'd be okay with wifi on the trains (even the pay WIFI they've got), but I don't want phone service on the train because as soon as you hit "above ground" and everyone pulls out their phone and starts making phone calls it becomes annoyingly loud.