He doesn't seem to drive that fast, he drove at a constant speed only to stop at the very last second. Had he been driving too fast, he would have been right into the train and probably not here to post on Twitter.

The car didn't slow down even as the train was clearly visible. An attentive human would have not only seen the train but also the very obvious flashing lights and have absolutely not problem stopping in time.

EDIT: From the link, it seems FSD was set by the car to about 61~63 MPH on a 55 MPH road, which is the speed most people drive on ( https://teslamotorsclub.com/tmc/threads/ap-fsd-related-crash... ). Considering the conditions, it is a little fast, but not unreasonably so. It took about 5 seconds from seeing the train to the crash, most of it at constant speed, so, a distance of about 140m. At that speed, stopping distance is about 100m, enough to stop in time. And that's for the unlit train, lights can be seen from much further as they are all visible at the start of the video.