Well, here's why: Railworks divides stock and scenery assets into categories. Most of the default stock - which came with your copy of Railworks - falls under the "Kuju/RailSimulator" category, which is selected by default in the default routes. Non-default stock and reskins (including Tornado and Flying Scotsman, which might have been bundled with your copy) are usually listed under the author's or publisher's name.

Above, you can see that I've loaded Castle Rock, and that the locomotive list stops after the SD40s (red arrow).
Click the category selector icon (yellow arrow, left) to reveal the selector panel (right). Choose the author from the drop-down menu and then tick the box corresponding to the addon you want.
Below, you can see that G-Trax' Southern Pacific GS-4 Sunlight - in various liveries - is now available after ticking the appropriate category.

The tricky part can be determining which category a given addon uses. Often this is detailed in the corresponding manual. Otherwise, you will need to explore. One specific gotcha is that the old RSDL Class 66 and 08 addons are under "RSDL/RailSimulator". RSDL later became RSC, and later addons from them are much more obvious in name.