Hmm...perhaps if the US had annexed more of Mexico in the Mexican-American War (not Ameriwank fantasies, just the next layer of Mexican states down, Sonora, Chihuaua and so forth), could there have been any impetus to build another trans-continental railway through the southern states to this new formerly Mexican territory? I just say it because it seems like a project that would provide a demand for southern-produced iron.
Annexing the next tier of Mexican territory is entirely plausible, if anyone other than Trist is the negotiator (as has been raised before). I'm not sure whether that would directly affect the presence or absence of an *Birmingham in itself. Butterflies could flap into producing it, I suppose, but it seems a little too much like a second PoD for me.
Of course, the big argument in the antebellum USA was on which route should be chosen for the transcontinental railroad, not whether there should be two routes. One reason it was delayed for so long was argument over whether a northern or southern route was preferable. Or, more precisely, whether the North or South should get the benefits of the railroad. Neither wanted the other section of the country to have it, and so it didn't get started until the ACW.
A southern route which runs to somewhere in Sonora
might be more attractive, since it's shorter, but it doesn't link to California, which was the main aim. I'm not sure how that one would play out, to be honest.
That said, if the first trans-continental railroad does go ahead on a southern route, and if there's an *Birmingham around, then yes, southern iron would probably have a whole new market to play with.
Adding the whole next layer of Mexican states would have serious butterflies related to the number of slave-holding states and their balance with the free states in Congress. That could affect when, and perhaps if a Civil War would occur.
Perhaps, although to be honest I think that 1860 would be quite likely to break out anyway. The killer turned out to be bleeding Kansas, more than anything else, and that was an argument which was likely to break out regardless of events further south. (Since it was largely a result of the problems with Missourians wanting to head west but keep their slaves.)
They wouldn't need to go that far in acquiring Mexican territory. The Gadsden Purchase added far less, but it was considered enough for a southern transcontinental railroad.
The Gadsden Purchase is certainly enough for a southern route railroad. Things get more interesting if the USA acquires enough of Sonora to have a railroad running to a decent port there, though. The best port in Sonora is Guaymas, I believe, but that's rather a long way south.
Both 1860 Democratic Party platforms called for Federal aid in construction of a transcontinental railroad. By which they meant a southern route; most of them were fine with "internal improvements" that helped their part of the country.
Quite. The dispute over the route for a transcontinental railroad was one part of the sectionalist disputes which led to the ACW. Of course, it was far from the only one.
In practice, I suspect that getting a transcontinental railroad through on any of the proposed routes will be quite difficult until the ACW or an analogue breaks out. There's just too much disagreement for government action. While private companies may start gradually expanding their own operations, it will take a while for that to turn into a full railroad.
