Ever since 1884, when this railway line was built, it has been known that the ancient Romans had something at this site. But exactly what it was, and how big, has only gradually become evident, especially in the last five years of systematic excavation.
Mogontiacum was the Roman name for what is now Mainz and vicinity, and it was important to them particularly as a strategic location because of the confluence of the Main and Rhine Rivers.
In the 11th century, a historian named Gozwin wrote: "Hoc etiam astruunt adhuc superstites theatri ruinae, quod Romano more ad ludos circenses et theatrica spectacula constructum est".
As you can see, the excavation site begins right next to the platform of track 4, where an InterCity Express is going through (without stopping, of course).
There is only a wire fence separating the platform from the excavation site.
The site isn't exactly open to the public, but that's no problem since you can see into it through the fence from all sides. And they do give guided tours sometimes.
Update: This railroad station, which for over a century was called Mainz Süd (meaning Mainz South) has now been re-named Mainz Römisches Theater (Mainz Roman Theater).
Right next to the south station in Mainz there is an archaeological excavation site called Theatrum Mogontiacensium.
With the help of numerous volunteers, including local school children and also some of the Americans who live in the area, archaeologists are gradually uncovering the remains of a huge Roman theater that once had room for ten thousand spectators, which is roughly eleven times as many as can now be seated in the large hall of the State Theater in Mainz.
The wood and metal construction is meant to show the shape of the theater, and where the first six rows of seats used to be. The stage in Roman times was where the station and tracks are located now.