Some initial steps have been taken, starting with the gauge conversion of the route from Mahishashan and Agartala-Akhauri. But a decent amount of GC work is required in Bangladesh to establish a through route.
The pre-independence route was actually to Fakiragram and not Agartala. Agartala was connected to Chittagong but not to the rest of the country as there was no bridge across the Brahmaputra at that time.
The through route was to Fakiragram via Parbatipur, Mogalhat, Gitaldaha, Golakganj. This route also needs a...
more... lot of GC work and moreover, the route keeps crossing the Ino-Bangladesh border several times and that is discouraging IR to pursue it.
I think IR has offered to help Bangladesh railways in the GC work of the Dhaka-Akhauri route to connect to Agartala. The first steps have been taken from the Indian side.