Hi Ron,
Welcome to Trainz, if you haven't been welcomed yet!
Paul and Cascade have answered this very well.
The HP and HR are some European developers. HP is from HP-Trainz which is a group of German developers who made some awesome stuff including the Marias Pass route included as well as their own Marias Pass X which is pay ware. HR is from the Hungarian Trainz (HOTT - I think). Their stuff is great too, albeit a bit older. They made a set of very low detail, but effective track which works well in yards and places where you need lots of track.
Now, without registering your program online, we have no way of knowing what version you are running. TS12, the latest version, handles high polygon assets very well while the older versions balked. This isn't saying that high polygon numbers aren't going to bring your machine to its knees, you still have to use them diligently. The most logical practice is to use the high polygon, super detailed, really pretty stuff, in the foreground close to the tracks where you and everyone else will see the operations, and relegate the less detailed stuff for the background. This isn't much different than what's done in the movies.
Now on to splines. These critters are a necessary evil. Track is a spline object, so are telegraph poles, roads, fences, and other stuff we really need. What you need to do is choose carefully. There are some splines that will kill the frame rates, meaning you can bring a route driving session down to a slideshow. This is due to the way they are made. They too have polygons, but instead of loading the object once, the same object is loaded over and over again as a new instance as the object is loaded in the segments. This is why there are stutters as you move along. There are other causes of stutters, but splines can really cause them. What I have done is use a few fences here and there, avoid the grass splines if I can which can cause awful pauses.
I seriously recommend registering. I have to say this as it is important. If you have registered previously, or are having troubles registering your serial number, please contact the helpdesk. In the even you are using a second account to post here, the helpdesk can take care of this too and merge your accounts together. Without showing a registered version, you'll be repeatedly hounded, and will receive little support as this usually indicates software piracy and we take that seriously.
John