we drive through tunnel usually 4 times a year - going this weekend in fact! it is very easy and you cant really go wrong. You do always stay in car but they do randomly check on vehicles, if you get chosen (which we have a few times) then things have to be unloaded, they look under bonnet, in boot etc. they do wave a detector around the vehicle sometimes and on our last trip back they had alsation sniffer dogs looking for explosives (it was in july just few weeks after the glasgow airport trouble). But as we have nothing to hide i dont worry about it but i am pleased such security is in place. As far as when on the 'train' which is in effect what it is, you can get out of your car, there is a toilet (although ive never used it) and you can go from carriage to carriage. Give it a go im sure you will enjoy!