while i hear what you are saying, rio can hardly be a trademarked WORD, maybe a logo, but it is spanish for river, who teh heck can you trademark that? the logo yes.

the word riocar, yes however, if they really wanted to put up a stink, all they have to do is say they are making a rivercar, and you will have a prb getting it without paying.

The chance of course to win, would be surround them with lawyers. In the long run, it is probably cheaper to pay them $5,000 and be done with it.