With SN your only real risk of consequence is being found and saved and admitted to psych. Give yourself a few hours alone and you'd be fine.
With a train... First consequence is that you fail, which could leave you with brain damage, missing limbs, a whole range of permanent damage. You say you're not worried about permanent damage, but no one wants to end up paralyzed or in a vegetative state - Then you really would be trapped. If you somehow came away unharmed, you'd be risking getting arrested (since being on the tracks is considered trespassing), and then also admitted to psych. And on top of all of that, you'd be causing trauma to the driver and literally anyone else on the train. Wouldn't recommend this method in the slightest.
If there's even a tiny bit of you that wants to seek help, seek it. Don't make a permanent decision like this if you're not entirely sure. There's people you can talk to and things you can say that won't automatically hospitalise you. I mean fuck, I'm UK based so it might be different, you could tell any professional you see that you're gonna off yourself and you'll be lucky if you get anything more than a brief conversation let alone hospitalisation.