Well, there are a few ways you could go about this.
One way would be to re-shape your underlying data to exist instead of the full-ride-per-line level to have each ticket split into 2 rows and have an additional dimension field specify "Departure" or "Arrival."
Within Tableau, if you don't have TOO many stations, another way would be to create a calculated field to act as the revenue measure for each Station.
e.g., London Revenue =
IF [Start] = 'London' OR [End] = 'London'
THEN 0.5 * [Price]
Repeat for each station, swapping the criteria for Berlin, Paris, etc.