Is Tantrayana the same as Vajrayana?

I'm reading a little bit from the Tibetan Buddhist lama Dhardo Rimpoche at the moment. He keeps referring to Tantrayana not Vajrayana when discussing Buddhist philosophy and practice. It's pretty obvious from context that this is the diamond vehicle i.e. Vajrayana. However I'm not clear whether the two terms are the same and interchangeable. Would Tantrayana refer to non Tibetan Vajrayana schools such as Shingon ? Is Tantrayana a subset of Vajrayana or is it a term only used by certain schools? Do they have a different nuances of meaning or are they genuinely exactly the same?