This initial prototype version was created using boards and tokens produced by thegamecrafter.com and cut/laminated paper for the private companies and extra tiles. There were 35 copies created and sold via the yahoo 18xx group and at a few conventions before the All-Aboard Games version was published. There were a few rules differences between this version and the AAG version. The All-Aboard Games version is the canonical game.
The primary difference in rules between the first and second editions is that the train manifest and exporting rules were changed. The original version of 18USA used the same exporting rules and trains as 1817. In the final version of the rules, 18USA adds two 3+ trains and replaces the last 4 train with a 4+ train. After operating round 1b, all 2+ trains are exported if any remain and phase 3 begins. After OR2a no trains are exported, and then after OR2b all remaining 3 trains are exported.
Additionally pullman cars were improved to give +20 per city instead of +10 and the Resource and Free Station subsidies were slightly improved. The $20 terrain cost West of New Orleans was also removed in the final version. The "Extended Game" rules were also removed in favor of always using them for games with 5 or more players. The Denver metropolis was changed to be oriented when a company IPOs in the city or when a company lays track connecting to the Denver tile rather than having a fixed North arrow to slightly improve the value of the Denver starting location. Any other changes are minor or simply rules clarifications.