Hi,
It is my understanding that it could have traveled into St Louis (IIRC) over a dual gauge bridge.
There it might have been loaded onto a SG flat (the reason I brought up this question) for transport to the Texas and Saint Louis for travel to beyond Houston, TX (sealy) or to beyond Waco (Gatesville). Both Houston East and West Texas and T&StL had planned extensions to Laredo, TX.
If the HE&WT had built to Corpus Cristi, the Texas Mexican railroad had a line from there to Laredo.
Back to St Louis, interchange with the StL&C (or C&StL) could take direct interchange in East St L to Cairo then by river ferry Bird's Point MO and the T&StL and then out to Texas.
Much of this is covered in Hilton's American Narrow Gauge Railroads. I think the NG car on SG flat was in an article in MR or RMC (since about 2000) about the transfer avoiding the StL&C.
Doug vV