My first choice -- Because I love this person so much, I find a way to save both.
Let's think this through -- What kind of train is this and what kind of tracks are these?
Bullet trains wouldn't give me that kind of access to a lever (not without guards, and even then, that's time wasted to save someone). L trains (like in a subway) are easy to get out of and you can step to the side on a ledge (at least you can with Chicago L trains). I can only assume this is a stream train or a freight train. The former is a bit faster, so I've got some worries, but the latter is generally slower.
Now let's think about the tracks -- do they easily snag clothes? Are they nailed to the ground or have an opening under it?
If we're talking tracks that can easily snag clothes, we're ripping the clothes off to save my love one. Plain and simple.
If we're talking about tracks that snagged a leg or whatever, specially a wooden board, I'm breaking the wood. If it's iron but my loved one can fall through to an underground level, I'll push them through and then pull them out. Otherwise, guess that's one broken ankle.
This isn't morals playing, this is just how much I love that person.
I won't let my loved one see me kill hundreds of people, thus traumatizing them and possibly losing their love nor will I put that type of guilt on their shoulders. And I am not putting hundreds of strangers I don't know before the person I love the most in the world, thus betraying my own heart and feelings.
Also, unless this is a bullet train or a durable train, everyone dies anyway if I don't save my loved one. Be it body or object, the moment something derails a train, it's game over. So then I would be both traumatized that I betrayed my loved one for nothing.