I remember we should get the red livery for the 628 when TSW4 came out. At the moment you can choose the green-beige livery only. The 628 arrives in red but as KI units at Lindau station only. About Maintalbahn I don't know. Because I don't own this route.
The 628 is AI-only on Voralberg, just moving scenery for Lindau bahnhof. That's why you can't choose it.
The red 628 was added so that future German routes set in present day that use the 628 in real life can be layered. Like the Voralberg and Maintalbahn as an example