I read that in the cannon scene where Keaton used too light a charge, the shot squibbed and landed in the engineer's cab, alongside of him. The commenter noted that it was an actual charge, and if it had been miscalculated, the ball would have killed Keaton. (no actors' unions in those days).
I groaned though, when the bridge collapsed and wrecked that great looking locomotive. And, in those pre-CGI days, they used one Helluva lot of extras with 45-70 breechloaders (current price $1,000).