1000mm is the common gauge in South East Asia, while 1676mm gauge is the common gauge in South Asia. Extension of 1676mm gauge network in Bangladesh means it's becoming a part of South East Asia. Considering cultural background, Bangladesh had better be a part of South Asia.
But, whether it's true or not depends on how high speed rails in India will be built. Since most of the high speed rails around the world adopted 1435mm gauge, I'm not sure that high speed rails in India adopts 1676mm gauge.
Double track to Chittagong
http://www.railwaygazette.com/news/single-view/view/double-track-to-chittagong.html
