Bengal govt wants to have restricted passenger on board system, like ticket should be issues based on the seat capacity.
But Kolkata Metro does have that facility which can estimate which passenger will travel to which station. Moreover, no physical token will be provided. Only smart card will be issued. In that case, it it more difficult to estimate or control the passenger in each station to mitigate the social distancing. Thus Rail can not provide that level of infrastructure what Bengal govt wants.