Shitty Movie Details

He technically changed the train from carrying a weapon to where it would drug everyone in the city (except a handful of people who'd had antidote and a handful of people in gas masks, one of which was Ra’s al) to carrying the weapon off the train tracks and into a train crash, but:

• he's got a strong defence that it's justifiable homicide rather than murder because in doing so, he stopped a bunch of people from dying.
• Ra’s al intentionally created the situation where the train had to be forced to crash by putting a weapon on it and driving it to where the weapon would do the most damage, so put himself in a life-threatening situation.
• Ra’s al intentionally created the situation where he'd die unless Batman saved him by staying on the train after it started the process of crashing so he could gloat about a perceived contradiction in Batman's moral code that forced him to save his enemies even after they'd just committed a mass murder. As Batman should not have reasonably expected Ra’s al to remain on a crashing train to prove a dumb point, there's a reasonable claim that it's only manslaughter even if we pretend that the initial train situation wasn't of Ra’s al's making.