The easiest way to make it look real would be to film a real train and then use a model of the same kind of train engine. You could use a macro lens to try and get a good shot, but the camera elevation will likely be too high still (unless the track is elevated). Putting the actual person on the tracks would be hard with iMovie as well since I don't know if it has much (if anything) in the way of keying ability.
You may be able to simplify it a lot, including the comedic effect by using direct cuts rather than a zoom as well. I'd recommend you consider building tension by cutting back and forth between the person standing on a track and a real train moving down the track. Start with fairly slow cuts back and forth and then get faster and faster as the train gets closer. Then at the last second, cut to a shot where the model train runs in to the persons foot (it should be the first shot that actually shows the train and person together.)
This will build anticipation as people worry about what will happen before the sudden comic reveal that it was just a model train.