Douglas simply wanted a route that would be good for him and does not particularly seem to have cared which one it was.
OTOH, in 1853 Senator Butler of South Carolina is opposed to all of the proposed Transcontinental RR routes. He favors supporting shipping from the Atlantic ports, either around the Horn, down to Panama and then overland to the Pacific, or to southern Mexico and the proposed RR/canal route to the Pacific.
I have read a good many excellent books on those days. I don't know what you may have studied, but you appear to have a lot of gaps in your knowledge of what was going on in the 1850s. The political universe of "the South" was very different than the picture you keep presenting.
Somehow, for example, you are portraying South Carolina as a great supporter of RRs. In real life, whatever the rhetoric you are reading, South Carolina was one of the least supportive States for RR construction to interconnect the states (in particular, to connect the USA -- Calhoun supported RRs to connect the slave states, not the whole country.)
Although James Gadsden was President of the South Carolina RR for ten years, construction of track ended in 1848 and nothing new was added until they finally built a 1.8 mile connection across the river to Augusta, GA in 1853. That was the end of expansion for that RR.
Another South Carolina dead-end to consider is the Blue Ridge RR, which died in 1858-59 when the legislature refused to extend funding. South Carolina never did build a connection to the West, contenting themselves with connecting to the RR in Georgia.
It should not be forgotten, however, that the northern route to the Pacific was being surveyed and had influential champions. Douglas himself had probably switched his personal interest to this route. He, Bright, of Indiana, Breckinridge, of Kentucky, and several other prominent Democratic politicians had acquired an extensive tract of land at the western end of Lake Superior. which was expected to be a terminus of a Northern Pacific railroad. Douglas had taken great interest in the survey of the northern route.Colonel Isaac I, Stevens, who was in charge of it, conferred with him as to route and reported his success. pp164 This is in 54
The immediate purpose and larger implications of the Kansas-Nebraska bill certainly did not escape Southerners outside Congress. Albert Pike, of Arkansas, stated them in his speeches and addresses in behalf of his plan for building a Southern Pacific railway. "Not content," he wrote, "with the natural and regular growth towards manly stature of the great country lying in the North-west, they have resorted to the system of forcing, as men use hotbeds in horticulture; and we see new territories of vast size and comparatively unpeopled, organized and established on the line of a Northern Pacific-Railroad--Oregon and Washington standing on the shores of the Pacific, and Nebraska and Kansas on those of the Mississippi--each clasping hands with the other on the slopes of the rocky mountains. It needs no prophetic eye to see in the future a cordon of free States carved in succession off from these territories, extending with a continuous and swarming population across the continent, giving such power of the Northern vote in Congress as has hitherto been only dreamed of and securing to their road, the Nile of this new Egypt, aid from the National Treasury, and countenance and encouragement from the general government." pp166 This in 54
The four Northern members (Douglas, Bright, Seward, and Foot) and Gwin agreed that the eastern terminus must be somewhere on the Missouri River between the mouths of the Big Sioux and the Kansas (Sioux City, Omaha, St. Joseph, or Kansas City). Within these limits the choice was to be left to the contractors. pp227 This in January 59.
Douglas and others drew an argument from the recent "Mormon War," which, it was estimated, had cost the government $5,ooo,ooo. If there had been a central Pacific railroad, the said, either the rebellion would not have occurred or could have been crushed in much less time and at far less cost. pp229 This in January 59
From Improvement of Communication with the Pacific Coast by Robert R Russel
Douglas voted for the Gadsden Purchase and for 3 Pacific Railroad Routes. Probably more to placate the Southern Democrats. However he Fought for the Kansas/Nebraska Act. The only way to get a Railroad built in that area was to Settle that territory. He blew up the Missouri Compromise to get the K/B Bill passed. Indirectly blew up chances of it Passing because of it. He wanted a Western Confederacy. A Northern Route of the TRR.