There was no actor on the train. Here's some background on the filming of the scene according to Buster Keaton: Cut to the Chase by Marion Meade:
During the filming of this spectacular scene on Friday, July 23, Cottage Grove shut down as businesses closed their stores and declared a holiday. In the hot sun, an estimated crowd of three to four thousand assembled. The scene was scheduled for 11:00 a.m. Six cameras were in position when Keaton changed his mind and ordered them moved. The crash train then practiced several runs across the trestle while the specifics were worked out by the cameramen and an explosives expert. There would be no second takes.
It was 3:00 p.m. when Keaton finally gave the signal. The engineer who started the Texas rolling leaped down, leaving behind a dummy at the throttle. The timbers of the bridge had been partly sawed, and when the dynamite charge went off, the bridge snapped in half. The engine dropped into a twisted, steaming pile in the river. Because the dummy looked so lifelike, spectators began screaming. The crash set off the train whistle, which continued to screech. Afterward Keaton posed for still photos standing on the wreckage. He was as "happy as a kid," reported the Cottage Grove Sentinel.
The train-crash cost came to $42,000 ($1.7 million at 1995 price levels), making it the most expensive single shot in all of silent films. That figure did not include disposal of the wreckage. Not until World War II would the Texas's rusty carcass be salvaged for scrap metal.