The server might have been S2 if all the cars were unlocked (and not just the S1 cars) or another parameter which might have made it S2, even though the track was SO.
From what I hear, cargame.nl S1 (and S2) server are pretty much the only highly populated public racing servers these days. There might be other servers, but they might not be as populated as cargame.nl...
The XFG(XFR?) and XRG might the only two you could drive at first, because it operates on ranking system. Since you don't have much online experience in licensed servers, you will have to start off with these two cars and work your way up the ranks to be able to access other cars... This is a third party restriction put in place by the server owner and NOT something from LFS itself.
I do understand that it is quite dishartening not to find any active servers, but you can try your luck at cargame.nl and work through the ranks... You might be able to learn the S1 tracks using the slow(er) cars, finish races to get points and move up the ranks...