The fictional scenario game I describe is set in a little later period than you ask about, post American Civil War. However, you may find some elements of use in a game about intervention in the Texas Revolution. With help from a friend we ran a 28mm Wild West/ACW skirmish games using Brother Against Brother rules.
This game included post-ACW (renegade) Confederates, Quantrill and his band, Bloody Bill and friends, Apaches, U.S. Cavalry, Imperial and Republican Mexicans and French (in Mexico, American Southwest). The Quantrill and Bloody Bill figures are all from Wargames Foundry.
This game include a pueblo town with a steam train, railroad tracks, water tower, and around ten buildings with corrals, etc.
Post ACW, the French are planing to sell the Confederate renegades a rail car mounted heavy cannon with the train for stolen gold. The rendezvous for the exchange is a rail town south of the border. The Imperials and the Republican Mexicans want the gold. Quantrill and Bill want the gold. The Apaches want the horses and cattle in town and guns and gold. The U.S. Cavalry wants the Confederates, the cannon and the gold. All the players have a different agenda/objective, alliances are encouraged between players with a lot of cut throat activity seen between players.
The train arrives early and the attacks begin at random from random/assigned directions. This game is played on a 6' by 12' board of southwest terrain. Photographs of this game scenario are on LAF if there is interest. More information and pictures at:
http://leadadventureforum.com/index.php?topic=49453.msg578547#msg578547I chose the Dixon 28mm train for my games. They offer a locomotive, cars and passengers in their Western range. They sell molded track sections, but I used S Gauge, model railroad track instead. I also added actual model railroading, S gauge wheel trucks to the rail cars, instead of the supplied assemblies from Dixon.
Using this train for this Southwest US/across the Mexican Border game scenario, I built a model of a large caliber ACW gun on a rail car (28mm) and used it in a skirmish game.
The shield on the gun car is built of railroad rails. The flat car is scratch built to fit on S gauge tracks. The gun is from the Huston Guns now marketed through Old Glory.
The figures in the game are from Dixon, Foundry and a few odd others. The cannon is from Houston's Miniatures, most recently sold by Old Glory.
Good luck with your efforts!