Well one can plainly see that the speed was consistent throughout the whole film sequence, so if they ran 4 mph over the bridge, they ran 4 mph for the whole time. At 4 mph, I think all those excited passengers would have been asleep. They don’t look like they are on a 4 mph ride.
A friend of mine who is very knowledgeable about movie making told me that it was standard practice to film at 16 frames per second in that era. Then later, it was standard to run those films at 24 frames per second. So if the train in the video clocks at 30 mph, it is likely running at 20 mph. I think the video is certainly showing 25 mph, so that would translate into 16.5 mph actual speed.
You can see one car pass the start of the trestle in about one second. If the car is 40 ft. long, that is 40 ft. per second. That is 27.27 mph. That translates into 18 mph when factoring in the film speed discrepancy.
So conservatively, I think it is fair to conclude that the train was going at least 15 mph over the bridge.