There are at least two places where one can watch Bangladeshi internal train services from India,
The best-known place is Hili, which features in a number of Youtube videos.
For more about Hili and its trains, you can see this: https://abn397.wordpress.com/2019/10/17/hili-revisited-2/
On the other side of Bangladesh, there is Quasba station:
The location can be seen here:
The peculiar border at Hili station came about because the Radcliffe Commission wanted to preserve the integrity of the main north-south BG line in East Pakistan.
At Quasba, it is a different story. The old Assam Bengal Railway did not wish that their lines entered the princely state of Tripura, so their main line going south to Chattogram ran within a few hundred metres of the border. The station was earlier known as Kamalasagar, a small town in Tripura. Its name was changed soon after it became part of East Pakistan.
One common attraction for Indian visitors to Agartala was to look into Bangladesh and see trains running through Quasba, which is a few hundred metres from the Indian border. In recent years this area has become a “soft border” where people from both countries can interact in a buffer zone. This seems to have benefited local businesses,
In these videos you can observe trains running in Bangladesh. All long distance trains from Chattogram to other parts of Bangladesh have to pass this way.
Pictures of these trains taken from higher points in India: