Multiplayer was a HUGE advancement in TRAINZ a few years ago when it came out, and as such, there were lots of routes with multiplayer sessions, and seemed to always be 10 or more sessions active at any time.
One of the biggest hinderances to that now, is that most content in routes is NOT on the DLS. For a multiplayer session to work, all content must be on the DLS. Unfortunately, there is a great deal of content that isn't.
I've been arguing for years, begging content creators to place stuff on the DLS so that it is accessible to everyone, and not having to go search for it - often times unscuccessfully. I've downloaded several routes that are really nice, but haven't been able to find all the content, which is extremely frustrating.
Some say that the DLS is way too slow, but I find it to be pretty fast and convenient, as when you download a route, the dependencies automatically come with it.
I love multiplayer, but there just aren't many folks creating multiplayer sessions, due to the lack of multiplayer-able routes due to content issues.