Chicago Express: The Expansions
Chicago Express has two expansions, which are available in a single box. Either expansion can be added to the base game individually or both can be played.
Narrow Gauge:Narrow Gauge adds 9 orange trains to the game. At the beginning of the game, each of the 9 trains is placed in a different city. When a player builds into that city, the train is placed in a community available pool. At that point, the train is available to be placed. To place one of the narrow gauge trains, a player may choose a "narrow gauge" action. Rather than having a limit to the number of "narrow gauge" actions available each round, a player chooses any of the "gauges" for the other actions and advances it one. In effect, this gives players something to do on what would otherwise be a null action. There is no cost to place an available narrow gauge railroad. The only restrictions on placement are that the hex chose must be a empty non-city hex and the hex must be adjacent to a hex containing one of the other companies. The effect of the narrow gauge RR is the same as if any other RR were in the hex: it increases the cost of the next train to enter in plains hexes and prohibits other RRs from entering mountain and forest hexes.
The expansion is fairly straightforward and easily implemented. It adds some value to "null" actions since you can now use the narrow gauge in order to make life harder for other railroads. I haven't played with it enough to really say with any certainty, but I've read some comments on BGG about it that it does make higher player count games more viable among experienced players (as in, it makes 5-player viable).
Erie RR Company: This expansion is a bit more interesting. The expansion adds the Erie RR company, which starts in Buffalo. Similar to the Wabash, the Erie is not available at the beginning of the game. It becomes available when a RR reaches one of the 4 RRs in the North/South column that has Cleveland in the Northernmost hex. The Erie is able to build into New York City (for +8 income) and is the only RR that is able to be able to do that. But it has a relatively limited number of locomotives (13) and can probably be more profitable by expanding west. The big kicker for the Erie is that it is only a single share. Because of the single share, a player only has one chance to capitalize the RR, so it can be a fairly large investment to win the bid. It's also a RR that will take awhile to reach its full income potential. Because it cannot be diluted though, it can be quite lucrative. The player that wins it will almost assuredly be playing a "long" game, so there will be a fairly large incentive to others to shorten the game as much as possible. (The rules do modify the end condition of the game so that 4 RR companies must deplete their shares rather than the standard 3).
I like the Erie expansion, it seems to add much more than the Narrow Gauge. With it in play, there is another level to the thinking of whether you're playing a short or long game and whether you should be capitalizing additional RRs or just sitting on your money and trying to end the game before the big income engines can be a factor. Because of the Erie's location, there's a potential synergy between the owner of the Erie and the owner of the NYC (Green). The two can potentially go through many of the same cities, which potentially doubles the value of any city improvement. Because of the potential large swing the Erie provides, comments suggest that it should not be used in a 3-player game.