Keevan,
The bridge shouldn't have a problem with a K-36 or K-37 (IMHO,) as we often run doubleheaded engines over the High Bridge on special occasions. The new bridge is reported to be five times stronger than the original. As Rick correctly points out, the problem would be clearance.
1)The "rock cut" is on a sharp curve and lined with 120-year-old rockwork, which is a historical landmark in its own right. It is already a tight squeeze, and I don't believe the cylinders on the Mikes would clear. 2) The "Pin Truss" bridge just west of the rock cut and the "low" bridge just west of Devil's Gate are both through spans on sharp curves. Same problem there. They'd almost certainly have to be replaced or widened for the big Mikes.
Face it - the Loop line was built for small Moguls and Consolidations with slide valves and narrow profiles.
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