Well at the time they were using the most massive equipment available to the point that a whole new class of steam shovels was created specifically for the project. They were rail mounted 105 ton (US tons and that is the vehicle weight not capacity) steam shovels. You can see one of the 6 prototypes for the project
here. It has a 2 1/2 cubic yard bucket instead of the original 5 cubic yard one (changed because the iron ore was substantially denser) and was also converted to crawler tracks to run in the iron mines of northern Minnesota but is the only remaining one of the prototypes. While this shovel never worked on the Panama Canal the only other surviving example of this type of shovel that may have is in
much worse shape and exists in upstate new york. They were built on a 40' railroad box car which houses the boiler with an additional 8' added on to the back for a coal hopper with the boom and arm attached to the front.