When I get over the channel from London to Brussels( just stopping for a few hours ) , should I have Belgium as one of my country's on my pass ? I'm going to go from Brussels to Amsterdam, Paris,Spain . Or will I need to have Belgium on my pass if I'am going to pass back through ?
For the purposes of rail passes, Belgium, Netherlands, and Luxembourg are all lumped as one country, called Benelux. So, if you have Netherlands, you'll have Belgium, too.
Also you need to be sure that the rail pass will save you money. It is not a given anymore that a rail pass is cheaper. You might want to read the other postings about passes.
3rd party passes for Benelux are almost never a good buy- ticket prices are cheap enough that it is impossible to justify the cost of a pass. And if your party will take more than a few trips by train within the countries, you can buy passes from the rail companies that are an absolute bargain compared to those sold by RailEurope and the like.