It really depends on a lot of factors. The traffic levels will effect whether you even have enough seniority to work. When the railroad is moving a lot of stuff, they need more people. When things get slow, they start furloughing guys until it picks up again. There are natural cycles to traffic levels throughout the year.
Where you work also comes into question. I know you said you are applying to Creston, IA. I am not familiar enough with Creston to give you a good idea. Terminals with more regularly scheduled assignments will obviously give you more opportunities to work a regular schedule. Some terminals have tons of regular assignments, others only have one or two. The employee retirement rate and turnover rate can also have an effect on how fast you gain seniority, which will put you closer to a regular assignment.
I worked for BNSF for about two years. At the end of that two years I had enough seniority at my terminal to hold a regular yard job in the summer, when there was a lot of traffic. In the winter though, when things started to get lean, I could not hold a regular job. That was in Glendive, MT, which is near the big oil rush. Because of that there was a lot of growth during those two years and lots of jobs.
Another thing you will find is that you may be able to hold a regular job for a couple of weeks, and then get bumped. Sometimes it is a nice break from being on call, even if you know it will be short lived. Last year I decided to leave BNSF and go to Alaska Railroad, and in doing so, I gave up all my seniority. Changing employers meant I had to start at the bottom again. That said, when I got out of training on Alaska Railroad, I immediately marked up to a daytime job with weekends off. Needless to say, I was quite surprised! I expected to last about an hour on that before getting bumped, so I was even more surprised two weeks later when I was still on that job! That particular job lasted about three weeks for me, which was about 20 days longer than I expected. I was able to hold a regularly scheduled job all summer, but not always the most ideal schedule. Several times I let jobs scheduled for the middle of the night go to lower guys and stayed on the extra board, because the money was better and at least there I had a chance to work during the day!