I've asked the same question in this forum and have also asked RSC, getting essentially nowhere with either. More than a year ago, RSC started offering USA and European (German/British) rolling stock packs; they now offer a "USA Ultimate Collection" and a "Europe Ultimate Collection", but these new DLC packages are just accumulations of route, engine, and freight/passenger equipment. This sorry situation is complicated by the fact that the same freight and passenger cars tend to come with a variety of DLC packs. I think it's safe to say that "default stock", meaning a body of rolling stock that everyone who has purchased the basic sim owns, no longer exists.
Like you, I suspect, I'd like to write scenarios that require a minimum of DLC purchasing and are thus playable by the greatest number of TS fans. The best idea I've come up with for achieving this is to write my scenarios using only the rolling stock available with the route where it takes place and a few items that have been around for a long time and thus strike me as "default". Once I've completed the player train's instructions, I perform an upload test to Steam Workshop, going as far as completing the upload if necessary to discover whether I've used something that RSC/DT flags as an add-on. I make note of any that equipment, delete the scenario from SW, and rework as necessary. Then, as I add AI traffic, I perform the same test from time to time, trying to keep non-default stock to a minimum.
As a long-term project, it might be worthwhile to create a scenario using one of the "default" routes, if such a thing still exists, adding rolling stock a bit at a time and keeping track of what shows up ... nah! never mind ... I, at least, would loose patience with that after a while.
We're just left in the lurch and not even RSC/DT has a firm idea of which rolling stock originated where. Too bad, I think.
