I posted this argument elsewhere against the "fake/staged" stance:
Certainly appears staged, but consider the following:
1. The camera girl makes a few comments, the last of which ("are you okay?"), along with the fact that she was not laughing, suggests she wasn't in on it (or it was staged incredibly well, in which case they could have improved the guy's script). Yet, the way she started filming long before the encounter started suggests she was somehow related to the couple and knew he was going to do this.
2. There are no other videos shot by other bystanders of this particular event that have appeared on YouTube, yet the train just so happens to go in front of this particular camera girl.
3. The camera shoots very poor-quality video, most likely on a cameraphone.
4. Cadbury has openly announced that they had no knowledge or involvement with this video. Source
5. The guy paused his speech while the train was passing by in front of the camera, and continued as soon as it was out of the way (but probably still in the background somewhere), suggesting that he knew the camera was there.
Given points 1-4, it's very unlikely that the video is any kind of advertisement for Cadbury and that the train pulling through is entirely coincidental and unaware of what was going on (but yes, the message on the train is excellently ironic). Point 5 reinforces the possibility that the camera girl knew what he was doing, but point 1 shows that she probably didn't think that what happened would happen.
Given point 2, I would conclude that none of the other bystanders who may have shot videos concluded that it was staged and decided not to upload theirs out of respect for the guy on the floor.
So, sure, it could be staged, because most of it looks like pretty bad acting, but don't rule out the possibility that they are just that awkward normally.
As an aside, there is compelling evidence listed in the same source above that suggests the video was not actually shot in Dubai, but rather in Mumbai. Therefore, the person who uploaded the video was probably not the camera girl nor anyone involved in the incident. I don't know if this conclusion plays any role in an argument.