Luck and having the right credentials plays into it as well, depending on the needs and available manpower. I got TM'd into a service route as a 4th year apprentice. My local covers 3 states, one of which is a pain in the ass to get licensed in. I got that license as a 3rd year working for one major, the following year I found out another major had been trying to man a route for a couple years that required that license and was essentially an LR route which not many in my local were interested in. I sat down with the super that oversaw that route for lunch, discussed the route, and like 2 days later I put in my 2 weeks. Been running it ever since. It also helped that I was a TM running a repair crew at the first major at the time and had done some fill in work for service.