I don't know what the attitude is within N3V, but from an outside view the public beta for SP1 was a disaster.
Releasing early versions for previewing is one thing, but a beta program is (or should be) an entirely different beast. Testers need to be qualified (and that does not mean on a technical basis), their continuance needs to be based on the quality of their reports, there should be no input to QA other than through formal bug submissions (discussions in the forum are just that - discussions) and there needs to be an accessible and regularly updated list of identified issues and their current status.
Preferably, testers should be assigned tasks (although that could perhaps be done informally amongst the testers themselves, or by selecting from a pool)) and there needs to be as much feedback about what worked as what didn't. There should be broad user input into what needs testing - it is too easy for overlook important things. And, most importantly there should be NO upgrade path from any beta version - every new test must start from the current released version, with a properly prepared update file (and yes, that would have an impact on who can afford to participate). There needs to be test suites (eg, KickStarter County) which are updated to demonstrate each problem and the fix as it is implemented (this sort of happened, but not in a very orderly fashion).
Typically, the effort put into properly managing and controlling a beta test program is returned many times over.
I'm not sure if you have done any beta testing for Trainz, but I have and I'll explain what happens and expected?
When I was in the very first beta testing for Trainz, Auran had an internal set of beta testers and an external set of beta testers, I was part of the external beta testers. Anyone that is an external beta tester shouldn't expect to be testing every little thing as the internal beta testers are testing the main parts of the program and the external testers are more to test on all sorts of computer systems and parts and test that parts are working properly.
You would do a list of things to test in the beta (to run though over 5 to 10 times) and if you didn't do the tests you generally would end up left out for the next round of tests. Also if the beta testers can offer any sort of added things that should be added that people would expect then you should put those in as well.
You'd log into 2 separate places 1 for bug test forum and 2 a beta download/test list place.
Testers are expected to test everything for example all the menu's all the content does in fact load up etc. of course some people aren't going to load up say all 500 items and so either say they didn't test everything or lie about testing stuff.
The beta downloads can't work like you say because they run from a dev version of Trainz and you have a Stable release version of Trainz, the Dev version can be many versions beyond the current stable version so you can't really go from the current stable dev to a beta version.
For example the versions I tested of Trainz and I still have some of the Dev/Beta releases, is 0.38, 0.48, 0.52, 0.58, 0.78, 0.81 and some more as you can tell these are before the first stable release of Trainz many years ago.
As for the content sometimes it can be tested and sometimes it can't for example kickstarter country can be officially beta tested but the C&O TANE content won't be as it's 3rd party content sometimes it's added in for testing but not always.
Generally only expect stuff made officially to be tested not everything.