I think this is a bug with the sequencer, I get jerky rendered animations when I add a “Look at track” with a Camera Rig Rail (following this documentation Camera Rigs | Unreal Engine Documentation )
on a PC, Win7, Titan Black, 32Gb RAM
Here it is without a “Look at track” but on a Camera Rig Rail
And here it is with a “Look at track” on a Camera Rig Rail
And here it is with a “Look at track” but not on a Camera Rig Rail
Hi twistedpancreas,
Thanks for reporting this. This bug has been addressed in this thread and you can track the bug here: UE-33632
Thanks , yeah I had “Force Fixed Frame Interval Playback” off for those rendered videos.
But where do I change the Time snapping interval to 0.1s like Pseudor says in that post?
It’d be good to get it sorted because I’d like to start using UE4 as my animation rendering software.
Time Snapping Interval is the white dropdown at the top of Sequencer.
Hmm turning them off and re-keying the Rail positions didn’t change anything either 
I can’t confirm whether that actually works. That was just one person chiming in on the thread. Can you download 4.13 Preview and confirm if this still occurs?
Ok 4.13 seems to work, although now when I key frame a beginning and a end “Current Position on Rail” and scrub on the timeline I get no preview of either the camera moving on the Rail or the Camera viewport moving on the Rail But when I render the animation it renderers it correctly (ie not jerky). So once issue seems to be fixed but has been replaced with another.
I’m investigating something similar here: Bug: Camera Crane breaks camera - Cinematics & Media - Epic Developer Community Forums