Some guys are lucky and hire out at the right time, and never get cut. I've been on two railroads, and was cut out of a terminal a few times. Once I was totally totally cut off for a couple of weeks once. I got back in an unfortunate way though. I was the senior guy cut. One engineer had a massive heart attack, and died at work. This got me back to replace him. The next guy below me was off a couple of months, so if not for the unexpected death, I would have been off that long too.
Look at the size of your seniority district. Some are small. Sometimes road and yard are separate seniorities, with no reciprocal rights. Go where you have the most options. Big districts give more options of places to bump to, if things are bad.
This is why I encouraged you to have a backup skill. I work with a guy who is a professional chef. A few times he got the old knives out, when cut off. Another guy was an electrician. He had better benefits on the railroad, vs. his own business. He had the skill though, if need be.
It sounds like you are fairly serious about giving the railroad a try. You may or may not like it. If you don't try, you may wonder what could have been. It is a good career, but the first few years are rough sometimes. This has been my point in getting you to set up for tough times, or an out, if you don't like it.
You are young enough to learn a trade, go to college, or even enlist full time. You can still get a full pension at 60, from age 30, if you don't loose any time. Why not be a kid for a while and learn. You may find a career which blows this away. The main point is to be prepared for everything, and also to keep yourself more marketable in the workforce.
My last point is that the military is a good starting point, which you have. Railroads like people who have kept large commitments, since you put in so much more than a 40 hr. week. Besides the military, college, and long held jobs show this. Most 18 year olds just have had a part time job somewhere, which is not enough commitment to show a good track record, which they require.