I am pretty sure that próximo works just as well here instead of siguiente. See: https://www.duolingo.com/comment/2606940/Spanish-help-Siguiente-VS-próximo. But please correct me if I am wrong.
I came here to post the same thing.
I want to use próximo but am not completely sure. I’ll ask my profesor and report back.
Update: Apparently, siguiente is proper here because próximo would be used prior to the word tren.