No, looking at the beta footage, that is clearly not the case. I would like a definite answer here, as it will effect my decision to buy the game. Will trains be able to choose a different platform if their normal one is blocked in the release version?

the train will simply wait a couple of seconds until the platform is clear and then it will proceed.

I think thats a bit harsh criteria, let the train wait a few seconds or build a multiplatform station, so multiple trains can drive in.
And I think, it stands on the todo-list, a 5-Men-Devteam builded this beautiful and really good game, to say “I don’t buy it because trains can’t chose different platforms yet” is like to say “I don’t buy that pizza because it lacks one of those many salami pieces on it”.
To make it short: You would regret it, if you don’t buy it since you would miss one of the first very good games in this genre since the RT3.
In Germany we got a saying: “Good things take their time” and this is especially true here. I’m sure, they’ll add some of the little features like this one here, that people are missing, maybe even before release for the beta testers to test, only devs can tell.