Are the bus and tram services being used for local transport in your ‘new’ cities?
What seems most likely however is that the travel time from your ‘old’ to your ‘new’ cities is too long. What is the frequency of your single train?
Especially in the early game it makes much more sense to keep distances short, because those trains are pretty slow! If you have two different concentrations of cities on your map it might make sense to first create two separate networks and then join them together if you have sufficiently fast trains (or can run them frequent enough) to make long distance travel attractive.