Here is how we play all the Mayfair "crayon rails" games. The idea is to allow everyone a shot at all cards. It may be a little confusing the first time when just reading the rules, but we believe that the mechanics are easy once it is tried.
Instead of each player having his/her own cards, there is a public pool of cards. We originally had people "claim" cards from the pool, but in order to fix a couple of game balance problems we modified it slightly. Here's how it works, modulo some special cases at startup.
For N players, there is a pool of up to 3N public cards. At times there can be fewer cards available, down to a minimum of 2N. Any player may make a delivery on any card, whereupon the card is removed and (usually) replaced by a new card. (Note: We haven't played many large games with this variant; possibly the number of cards should be reduced for 5-6 player games.)
At any time during a player's turn, until he starts building track (or upgrades his train), he can "lock" a card for a particular demand. To do so, he must be carrying the necessary load, and the load must not already be locking another card. Once a card is declared to be locked,
When the locked cards stay public, you need some markers to indicate which player has locked which demands. Small flat chips work best; three of each color.
If a player is derailed, he can give up a locked load. If he does so, this unlocks the card (unless somebody else has also locked the same demand). Derailment and having somebody else deliver the demand are the only ways you can "free up" a locked load.
The limits on the number of cards are: There are never more than 3N total cards visible, and there are never more than 2N unlocked cards. Thus, if there are 2N unlocked cards and someone makes a delivery, no new card turns up. A few side effects worth noting:
When a player chooses his initial two cards, he must declare which loads he intends to deliver for them. He takes the loads and places them face down on his train; the cards are locked, and the loads are reserved for him, but he must still get to a city where he can pick up the load. When he does so, the load gets flipped over to show he's really got it. Reserved loads count against the counter limit in the usual way (two players can't both lock down 2 sugars). Reserved loads also count against the capacity of the freight, so the holds cannot be used for other loads until the reserved loads have been picked up and delivered. As with all locked loads, reserved loads cannot be voluntarily discarded. Reserved loads that have not yet been picked up cannot be selected for loss in a derailment. If the load for a privately locked card, after being picked up, is lost in a derailment, the player may choose to retain the card as a privately locked card (in which case he must as usual go back to pick up the load again), or let the card go into the public pool.
You don't have to choose two cards from your initial four. You can choose to take only one card, or none. Any cards you do not take are given to the next player, who still receives two additional cards and may then choose up to two from the combined set. So, though you might not want to commit yourself early to a particular pair of cards, you have to be wary of passing good cards to the next player in line. The last player may be willing to lock no cards, provided all players in front of him have locked two, because he can then wait to see any additional cards that turn up before committing himself. (He may have to worry about someone ahead of him upgrading to a Heavy Freight right away, and locking a third card before the last player gets a chance to move. Usually, though, nobody can afford to pay 20 for the upgrade and still build enough track to make any of the three deliveries.) If enough players choose not to take two private cards at the start, then it is possible for the 2N limit on unlocked cards to apply, and additional cards are not turned up until more cards get locked. (We've never seen this arise in practice.)
Typical strategic scenario: You'd like to lock down a tobacco load, but if you head in that direction, someone will lock the card with machinery. But if you don't head for the tobacco, nobody has to lock the card with machinery, and eventually someone else may get the tobacco (or perhaps the third demand on the card). So you sometimes find you have to build bits of track or tweak your movement in order to threaten to pick up an expensive load so as to force someone else to commit to stopping you, etc.
Finally, to prevent screwy end-game maneuvers, you cannot win if you have any locked loads. This means that someone who's in the lead can't lock down loads that he has no intention of delivering (he might not even have the necessary track!) just to keep other people from doing other loads on those cards.
Addendum: Pitching cards. A player who has no locked loads may give up his entire turn and pitch three randomly selected unlocked cards. The player may then lock any cards (including ones that did not get pitched) using loads he already has on board. He cannot pick up new loads at his present location that turn in order to lock cards. Locking cards may bring up additional cards (if there were fewer than 3N cards up), which may in turn be locked by the player who pitched.