How did the use of a standard gauge
for railroad tracks drive down shipping times and costs?

Originally, various track gauges were used in the United States. Some railways, primarily in the northeast, used standard gauge of 4 ft 8 1⁄2 in (1,435 mm); others used gauges ranging from 2 ft (610 mm) to 6 ft (1,829 mm). As a general rule, southern railroads were built to one or another broad gauge, mostly 5 ft (1,524 mm), while northern railroads that were not standard-gauge tended to be narrow-gauge. The Pacific Railroad Acts of 1863 specified standard gauge

Explanation:

the constitution gives congress the power to regulate commerce "among" the several states.

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