Strasbourg - Saint-Denis is a station of the Paris Métro, serving line 4, line 8, and line 9. The section of lines 8 and 9 from just east of Richelieu - Drouot to west of République was built under the Grand Boulevards, which replaced the Louis XIII wall and is in soft ground, which was once the course of the Seine. The lines are built on two levels, with line 8 on the higher level and line 9 in the lower level. The platforms are at the sides and the box containing the lines and supporting the road above is strengthened by a central wall between the tracks.