If you can shoe horn yerself on...................
"Signal Iduna Park can be easily reached by train from Dortmund’s main railway station in the centre. In just 5 minutes you will arrive at Dortmund Signal Iduna Park station, from where it is a short walk to the stadium. Trains typically run three to four times an hour.
Alternatively, you can take the U-Bahn from the centre and get off at station Westfalenhallen, from where it is a 5-minute walk to the stadium. Both line 45 and 46 serve the station.
On matchdays, there is a special service that will continue to station Stadion, which is even closer to the stadium.
The stadium is furthermore within walking distance of Theodor-Fliedner-Heim station, which is on U-Bahn line 42".