The pistol Brandenberg was shooting did not have a visible hammer. It also appeared too small to be a 1905 or 1911. It likely was a Colt 1903 Hammerless.
Colt produced a Browning designed Model 1905 pistol in 45 ACP, the 1911 did come after that. Per the Standard Catalog of Firearms 2010.
When Brandenburg is shooting from the back of the train, His first shot is heard but no muzzle flash is seen.
Near the end of the movie Augustus Brandenburg is shooting at the Cartwrights from the back car of his train. The gun he is using is a Browning M1911 .45ACP Designed in 1911 by John Browning. However the date for the events in the movie is clearly given as 1905
At the end of the movie when they are all sitting on horseback with Lake Tahoe in the background, the glare off a moving car windshield in the background can be seen across the lake.