Myanmar is a more interesting case. It was also a British colony and was at one time ruled as a part of British India. All pre-independence Railway board reports also contain information about railways in Myanmar( then Burma). And works were started at one time, to connect the Indian subcontinent with Myanmar with an MG line.
India's eastern border with Myanmar is completely hilly, with a very extreme terrain.This makes running any railway lines through Nagaland, Manipur or Mizoram essentially impossible.
The only possible route is through Bangladesh. The construction of a...
more... railway line from Chittagong (now in Bangladesh) was started at one time to connect to Myanmar. With this route, passengers would have been able to travel through Calcutta or any of the numerous meter gauge routes in Assam/West Bengal to the Western bank of the Padma river near Dhaka, take a ferry to Dhaka and then take a meter gauge train to go to Myanmar through Chittagong.
However, when this project was progressing, the world wars broke out and construction was put on hold. After the end of WW2, before construction could resume, India was partitioned and Dhaka/Chittagong went to Bangladesh, which meant a direct rail connection from India was no longer possible.
Even though we have the necessary technology now, constructing a railway line through the extreme hilly terrain of the north-eastern states to Myanmar will be an unreasonably costly venture with little to no benefits. So, it is not being pursued.