I have come to clarify the train!
You see, back in the golden age of the ocean liner, there used to be things known as "boat trains."
They were special trains that would depart major train stations (In the case of England, London Waterloo) and pull up, straight to the dock of whatever ocean liner it was arranged for! It was a service meant for first class passengers.
In the images of the build, the parallel gray bricks spanning the length of the dock, right next to the ship, are (placeholders for) where the tracks ran, as seen in the following image:

White Star Line's boat trains were run by London and South Western Railway, or LSWR (as seen on the sides of the coal tender, if you ever get a close look at it). The units of the train are recreations of units that I have been told are potential candidates for the consist of White Star Line's boat train on the morning that Titanic left.
So, yes, it's something meant for the dock.