Small triangle is going to be landscaped greenspace (see page 45) https://www.cambridgema.gov/-/media/Files/CDD/Transportation/Projects/obrienhwy/finaldesignplans.pdf

I don't think any structure has been approved for the rest of the site where the old station sat. I think originally it was going to be retail. It's in private hands now, so you'd think there is no way it would remain greenspace (unless Cambridge purchases the land back), but all I see is a huge mound of dirt being built up there.