03-25-2012, 09:24 AM
Height of US highway bridges is anything over minimum clearance, and that's changed over the years. In the 21st century, main lines that have double stacks need a clearance something in the neighborhood of 22 feet, and there were major projects in the 1990s to eliminate or replace bridges with clearances of less than that on Eastern main lines. There were equivalent campaigns in the 1930s to accommodate higher-height boxcars. In the steam era, clearances could be as low as 15 feet or so, and some locos, like the NYC Niagaras and PRR Q2s, got a certain amount of their look from having to stay within those clearances. Smoke was never really an issue, it was the size of the equipment.