I think you will just have to come along and check it out for yourself. I suspect that it is going to be heaving with people here. We used to get 10,000 spectators for the annual bonfire and the normal population of the town is under 5,000! I imagine that the TdF stage will bring out at least the same size crowds.
I am not sure that it will be easy to get on a train that day. I just looked on the National Rail journey planner and they are still only showing one train an hour! I would have expected them to lay on lots of extra trains. Maybe they just haven't updated the timetable?
There will be a big screen showing the whole stage in the park at Hebden Bridge, just a short walk from the station on the way into town. There is also going to be one in Mytholmroyd, close to the A646. (You can walk to Mytholmroyd along the Calder Valley Greenway from HB station if HB park is full - right as you come out of the station building. It is about a 30 minute stroll.)