When you say improving existing routes can you get rid of all of these entry portals and exit portals and replace them with terminus stations. having portals is a bit unrealistic.
In real world the continuous railway network is a large complex web that can't all be modelled as one route so you'll end up with some branches and stubs that lead to unmodelled parts of the network. These are often modelled as portals of some kind in the other train sims as well. Then again the current routes are fictitious so the portals could be replaced with terminus stations to form a small-ish but complete railway network. Another option would be to have the run start and end at a non-terminus station instead of a portal.