This is my second playthrough. The first time, I didn't want to kill the railroad. so I started over and I chose the minutement to build the contraption that will teleport me to the institute. I saved right before maxson promotes me to Danse's former title after saving Danse from Maxson. Right after that it gives me the quest to kill off the railroad. I loaded up the save before this happened and I started doing the minutemen quests. Is it too late to not figth the railroad?

No, minutemen is totally independent, you can go back to Maxson and he will give you that quest.