Farming rerolls?

I'm currently in the process of trying to go for a new high score using research body. I'm currently on wave 40 (crazy, I know) and wondered what would just happen if I didn't kill the boss and waited for cargo trains to pass by so I can farm them for rerolls? I'm pretty sure the game forcibly starts wave 41 after a certain time period, but I've been sitting here for around 7 minutes now and I already got a few trains to spawn. How long does it take for the game to advance the next wave even if bosses are still alive?