I know I've read about popular support in California a couple of times, whether for Union or Secession, but I can never remember exactly what I read
Probably because it was fairly unspectacular. I guess the population was mostly busy with getting on with their lives, although IIRC sentiments were fairly evenly split. I'm guessing the East was just too far away.
I once proposed a method for building new rail lines during the game, but it was deemed to be too difficult to implement, although IIRC it would have been done with only RGD's, and a couple of Surveyors units. Just now thinking about it, you could add a couple of disc-marker (you know those round icons denoting Siege, Iced-In Harbors, etc - make one showing a new surveyed region and a new railroad; easy-peasy
You would first have to plan the route, which would have to terminate in a city, or coupling with another railroad region. Then each region along the route would have to be surveyed before building could begin. You can only build in a region neighboring a current railroad region.
Of course it would always have to couple onto a previous rail line region, and would take a long time to build in a region. The more difficult the terrain, the longer it would take and the more expensive it would be. Crossing minor and major rivers would cost extra and also take extra time... and maybe it would take a little more than just writing a couple of RGD's, and adding a unit and a couple of markers...
On an historical side-note, the rail line between Keokuk and Des Moines, Iowa was actually only completed in August of 1866: How the Des Moines Valley Railroad Came to Des Moines PDF
, and Sioux City in 1868: History - Sioux City Railroad Museum
Also, the Des Moines River should be navigable up to Des Moines for steam boats, but be very susceptible to flooding and low-water blocking traffic, neither of which we have in the game.
You get conscripts if you hold Baltimore according to loyalty, the same as any other city. But if the South controls Baltimore, a small number of units should be added to their unit pool as well, which probably ought to only be built in Baltimore, or maybe also Fredericktown... (Fredericktown