A rail connection to the ASEAN countries could help Bangladesh export certain goods faster to markets via ports in Thailand or Vietnam, skipping the shipping time needed to go around the Malay peninsula. And via a verse, It would also make Bangladeshi ports (such as Chittagong) attractive to producers in ASEAN countries as well as southern China for export to the Middle East. Food stuffs (like fruit) in particular could be a viable exportable commodity with shorter travel times.
Good points and I agree. However we don't have a direct border with China - the rail route has to go through two problematic neighbors, India and Myanmar.
China's interest in developing the route is not there as Kunming already has a route to Myanmar ports, and India will block any attempt to connect to the Bay of Bengal through itself and Bangladesh (as it is opposed to the Chinese string of pearls or "Belt and Road" initiative) as they see it.
India and China relations are also problematic to say the least and trying to go through India and Myanmar to Kunming in China (or using the Myanmar rail route to Thailand) are both ridden with problems, as Myanmar as a country is unstable, the rail route in some areas is unreliable and will remain so for the near future.
The best option is to connect for container traffic in the Thai mainland port of Ranong/U-Tapao/Pattaya which is around 900 miles from Chittagong. Thais can then transport these containers via rail to other places in ASEAN as they see fit. Our private sea freight operators already do this for some China bound containers.
Going around the straits of Malacca and Singapore is not a major time waster as Chittagong has direct marine container connection services with ports in China, the ships don't stop in S' pore.