According to the
Wikipedia article on TGV, it's a French train network that has the fastest
wheeled train - in other words, it isn't a mag-lev train, just a really fast conventional one. The article mentions that they have prototypes for mag-lev trains only.
As for mag-levs in general, that's a good question. I wouldn't think that the magnetic field should impact within the train itself, based off of my shakey memory of general physics. From a practical viewpoint, it would make for an unviable business model. Everyone carries cell phones these days, among other electronics; nobody would ride the train if they couldn't bring their stuff with them
Edit: a
bit more research revealed that there are three types of maglev trains, one of which does subject the passengers to a strong magnetic field. The article specifically mentions items such as hard drives, pacemakers, and credit cards as being items that could not be taken on the train, UNLESS magnetic shielding were employed.