That will not look good, getting hired and then almost immediately asking to mark off!
Seriously! You see, you will be in the 90 day probation period during which you can be terminated for ANYTHING without recourse. All that it takes for that is for some bosss just not happen to like you, and you're done.
When we, the current and former railroad people, tell you this stuff about how rough the railroad life is, we are NOT kidding. Depending on the division, the railroad, even the particular bosses you have, marking off can be touchy subject with them. I saw my Terminal Superintendent scream out, "AWRITE, I f###in' DARE anybody to mark off right now, YOU HEAR ME? YOU DON'T WANT YOUR JOB, JUST TRY ME!!!!"
With that said, I can't see the harm in ASKING the boss about it; asking a question about it as a new hire and DOING it (marking off) is two different thing! You could explain that the opportunity to work for the railroad came along, and the event/wedding/activity had been long scheduled previously and you didn't know you would get this wonderful(?) opportunity to work for them. Most bosses---even railroad----wouldn't mind answering that question. You are, after all, a new hire who doesn't know such things. If the answer is not favorable for marking off, then simply reschedule the event.
In the early years in T & E service, especially, you are not going to GET much time off; that is, weekends off when most events and neat things occur. I know, most people are used to working a regular shift of 8-10 hours and going home. They can PLAN activities. With the railroad, it's gonna be different until you "grow some whiskers" (build some seniority), and you might as well get used to being "owned" by the railroad (Saint Peter don'cha call me, cuz I cain't go; I' dun marked up to the ##### railroad!)
Sure, you could get lucky and get the easiest Trainmaster in the world, OR you can get the worst sum#### that ever pounded ballast! I had several really good bosses, but they always say "nice guys finish last!" That's what happened: they always seemed to get transferred out pretty quick to be replaced with a really grumpy bruin! Got some of THEM, too!
Gud Luck! YMMV.
GF
Gadfly