$9 Udon from 2nd-22nd September at Sushi Train restaurants.
Looking at the website this includes the normal dashi broth udon and the curry udon with various toppings like karaage chicken, beef, tempura prawn and tempura veggies.
The tiny text at the bottom of the picture says participating stores only, but they haven't announced which ones will be participating yet.
What's the normal price?