In Begins Bruce refuses to save Ras Al Ghul from the train crash. On the other hand at the end of Dark Knight he saves Joker from a killing fall despite the fact that he caused him great personal loss. This seems like a contradiction. Why did he not save Ras too?

it was what both of them deserved.....but not what they needed

Where Batman would've been at fault for Joker's death, Ra's would have been at fault for his own. Batman was the one who endangered Joker, therefore, he had no choice but to rescue him. Ra's created the situation with the train. Batman had no intention of killing Ra's, but he also didn't have the obligation of saving him. He left Ra's in charge of his own fate.

@AweSam said:

Where Batman would've been at fault for Joker's death, Ra's would have been at fault for his own. Batman was the one who endangered Joker, therefore, he had no choice but to rescue him. Ra's created the situation with the train. Batman had no intention of killing Ra's, but he also didn't have the obligation of saving him. He left Ra's in charge of his own fate.

that works too

@AweSam said:

Where Batman would've been at fault for Joker's death, Ra's would have been at fault for his own. Batman was the one who endangered Joker, therefore, he had no choice but to rescue him. Ra's created the situation with the train. Batman had no intention of killing Ra's, but he also didn't have the obligation of saving him. He left Ra's in charge of his own fate.

basically this, I think they even kind of mention it or imply it in the movie

@AweSam: agree!