Unless you have already mentioned some day OTHER than the following one, 'in the morning' always means TOMORROW morning. Of course, if you have already spoken about 'today', ' last Tuesday', 'next Wednesday' or some other given day, past or future, you mean the morning of that particular already-mentioned day.
You might, therefore, say, 'I was in London last Tuesday. I went up by train in the morning.' Alternatively, you might say, 'We're visiting Susan next Wednesday. We're planning on driving over in the morning.'