I'm assuming this works pretty smoothly with the Rail Ring, correct?  Also, after perusing the guide, I didn't see (or missed) how to actually place a work cart onto the track.  Is this something this plugin does?  If so, how?  Thank you.

Yes, the plugin works with the current state of the procedurally generated aboveground rail network. The tunnel trigger feature does not apply there of course, but the map trigger feature does. You can think of the rail network as the same as any aboveground tracks of a custom map.

One of the best capabilities of the plugin is the tunnel triggers, because they allow you to set up routes that will function across pretty much any procedural map seed, since the spawn points are well placed, there are many loops, and double rail. That also allows the plugin to provide default triggers to get people started easily. Unfortunately, the current design of the rail network is very basic and does not have the same properties as the underground tunnel network. This means you have to set up all triggers manually per map, that there are no natural stopping points, only one direction, etc. I understand that FP is planning to address some of these problems. Until they do, I am not planning to spend any of my limited time on improving rail network integration. Hopefully the solution they end up with has similar properties the underground tunnels, though I am a bit worried about issues that the wagons may bring.

The plugin does not currently provide the capability to spawn workcarts.

Following up here for visibility, in case anyone reads this thread later.

The plugin has been updated to fully support the new trains, as well as the ability to spawn trains. It can even be used to spawn trains without automating them.

As for rail ring integration specifically, I'm still waiting for Facepunch to implement monument connections. Once they do, I will see if there's a reasonable way to implement monument-relative triggers (similar to tunnel-relative triggers) so that those triggers can be reused across many different procedural maps. Depending on how Facepunch implements monument connections, and depending on how long they take to do it, I may implement a feature to generate spawn triggers and stopping point triggers along the rail ring.