Where can I find all the Diamondbacks in the Great Train Robbery?

Solution 1:

There are 3 Defiler bones that are scattered across the map:

• One bone is located south of your starting position, and you can see it from your starting position.

• The second bone is located to the northwest of your base, near the Train tracks. The third Defiler bone is located directly south of your starting position, next to the bottom Train tracks. The first two are revealed on your mini map at the start of the mission.

• The third is revealed by the second train. In all cases, they are all revealed by the 5 minute mark, and none of them are guarded, at the beginning of the mission. Get them as soon as they are revealed, before it becomes harder.

Source: TL Wiki - Starcraft II - Campaign - The Great Train Robbery