Unfortunately, we have no idea when we're getting a second season. Considering the movie won't even be out in Japan until later this year, we won't be getting a second season until 2021 at the EARLIEST.
5/3: And if anyone else was curious, the season 1 finale leaves off part way into chapter 54 (collected in Volume 7) of the manga and the Mugen Train arc concludes in chapter 69 (collected in Volume 8).
(It also looks like the current tankobon collection is Volume 20. I think up to Volume 11 has been released in English. The latest chapter in Japan is 203.)