[1010RD]

At what point in time and by what cause were all the gauges in the US standardized?

[rockrr]

Interesting question. I had previously done some research based on BroJoeK's post of that picture and here's a bit of what I found:

Here's a quick history of rail-line development in the United States:

http://railroad.lindahall.org/essays/rails-guage.html

This link is a dry read PDF file but has more raw data:

https://campus.fsu.edu/bbcswebdav/xid-3430397_1

The bottom line is that the industry evolved, both north and south, starting with serving local interests and eventually recognizing the desirability of standardization for economic reasons, and all apparently done without federal interference.

Perhaps more specific to the decision point in the south, here is a quote from wackypedia:

In 1886, the southern railroads agreed to coordinate changing gauge on all their tracks. After considerable debate and planning, most of the southern rail network was converted from 5 ft (1,524 mm) gauge to 4 ft 9 in (1,448 mm) gauge, then the standard of the Pennsylvania Railroad, over two remarkable days beginning on Monday, May 31, 1886. Over a period of 36 hours, tens of thousands of workers pulled the spikes from the west rail of all the broad gauge lines in the South, moved them 3 in (76 mm) east and spiked them back in place. The new gauge was close enough that standard gauge equipment could run on it without problem. By June 1886, all major railroads in North America were using approximately the same gauge. The final conversion to true standard gauge took place gradually as track was maintained.[1] Now, the only broad-gauge rail systems in the United States are some city transit systems.

[BroJoeK]

1010RD: "At what point in time and by what cause were all the gauges in the US standardized?"

Yes, much is often made of the South's various gauged railroad tracks, suggesting this was a major problem for them.
But, as I read the map, that wasn't necessarily so.

I see only two major gauges -- 4' 8-1/2" plus 5', and I see where major Confederate cities were connected to both, i.e., Richmond, Petersburg, Norfolk & Wilmington.
So it appears that once freight was loaded on a particular gauge line, it could run hundreds of miles before having to change gauges:

• Wilmington to Richmond = 260 miles on 4' 8-1/2" gauge.
  
• Norfolk to New Orleans = 1,100 miles on 5' gauge.
  
• Savannah to Memphis = 630 miles on 5' gauge.

So, I don't see Southern railroad gauges as a major limiting factor.
What I do see are many other railroad issues, including:

• In 1860 many railroads did not go all the way through cities.
  It meant that rail-cars had to be unloaded at one end of town, transported across by horse-drawn wagons, and a new train reloaded on the other end of town.
  This made the particular gauges of each rail line somewhat irrelevant -- freight had to be off-loaded & reloaded anyway.
  It was most particularly a problem in Baltimore, leading to both political embarrassment for incoming President Lincoln, and the first actual deaths of the war.
  During the war, it would cause on-going delays in transporting men & supplies to the front.

• Rail and rolling stock maintenance. Especially after the loss of Cumberland Iron Works, near Nashville, the Confederacy never had enough iron to keep things running effectively.

• Union Army depredations of rail, bridges, engines & rolling stock grew greater with each passing year.

In short, while the Confederate rail system was not as extensive as the Union's, they had enough rail, if it could be maintained, to serve their purposes.
And, to my original point: Confederate blockade-running ships could therefore land at any of more than a dozen different ports, communicate via telegraph, and then transport their wares by rail to anywhere else in the Confederacy.
And that would render the operations (or non-operations) of any one port (i.e., Charleston, SC) somewhat irrelevant.