Nothing to Rail At
Unlike a lot of the copious quantity of reviews of Union Pacific here, this one will be from the point of view of someone who knows almost nothing of the pedigree of the game. I have not played Acquire or Airlines and so can't comment on Union Pacific's similarities. This isn't a problem as far as I'm concerned, and the game definitely stands on its own merits.
The map that you play Union Pacific on is quite arbitrary and doesn't bear a lot of resemblance to real American railroads of the time. (If this bothers you, there are plenty of historically accurate railway games around that will appeal to you more.) Indeed, you could probably put much the same connections reasonably onto a map of Europe or almost anywhere else. As a result, the game is just a little abstract - unless the railway theme appeals, in which case it's definitely about railways!
The different railway gauges - all usually mutually incompatible - are nicely handled in the game play, with some lines only able to use some gauges. The board is also very easy on the eyes, and the range of colours of the plastic train pieces good enough to make each distinctive. The tray that all the pieces are stored in is perhaps the best use of game box space I have ever seen.
The mechanics of the game play are amply described in other reviews, so I will just comment that Union Pacific is quite easy to learn (the rule book is delightfully clear) and has depths that are not obvious at first but remain to be discovered through game play.
Union Pacific is entertaining and has replay value. Definitely worth getting.