In my last four choo-choo games - Iberian Gauge (2017), London & Northwestern (2018), The Soo Line (2018), and Dual Gauge (2020) - the game's structure is bifurcated into stock rounds and operating rounds. In Iberian Gauge, a stock round will continue until all players pass consecutively, giving players multiple chances to buy shares before the next set of ORs. But with the other three games, a player only gets a single go during the stock round, in which they are limited to the purchase of a single share.
This does a couple of things. First, and most obviously, it makes that decision - which share am I going to buy - more meaningful. In Soo and Dual Gauge, in which the majority shareholder makes all decisions for a company, whether to use your single share purchase to double-down or to diversify is a key tension. If a company is unable to build track, it is forced to withhold, which can sink its stock value catastrophically. I don't want my company to be underfunded, especially if I'm deeply invested in it. At the same time, if another company is performing well, I'm going to want to get in on that. Because Dual Gauge is a share dilution game, I've also got to consider whether or not to purchase stock destructively - that is, making a purchase not because it benefits me, but because it harms an opponent's income.
This is complicated, as it always is, by the presence of the other players. Do I think one of them will do my job for me? Will Travis buy that share of red that cuts Bob's income by a third so I don't have to? Will Calliope buy a share of my underfunded blue company, cutting into my income but allowing it to build track this turn, while freeing me up to take over the majority shareholder status of yellow?
Turn order matters here, of course, especially as - common in my stock games - the purchase of a single share will immediately increase its value and cost, potentially putting it out of reach of players who act later in the stock round. In fact, aggressive and calculated stock buys can ensure that a certain player doesn't get to make any purchase at all. All's fair in love and locomotives, but at the same time one of the other reasons for the single share per round limit is that I wanted to prevent a cash-rich player from brute forcing their way into an insurmountable lead in the paper chase. Consider: if not for the single share limit, a player who has a lot of money at a moment in which their rivals are strapped for cash can swoop in and gobble up a series of shares on the cheap. A player who has eight shares at the end of the game when everyone else has four is highly likely to win it in a walk, even if those individual shares are comparatively less valuable than those held by other players.
Falling behind in the shares race therefore is catastrophic, and if players who are cash-rich happen to consistently act earlier in the round than their poorer adversaries, the latter are likely to fall behind; it's that brute force problem again. Allowing cash-poor players to act earlier in the round circumvents this somewhat - it can still happen, but of necessity is going to be less frequent. In London & Northwestern, a whole new turn order is determined each round, from poorest player to richest. Dual Gauge doesn't go quite that far: the poorest player has the first stock buy in the round, and it moves clockwise from them. It's still possible (and with poor play, even likely) for a poorer (but not the poorest) player acting late in the turn order to get caught without a share. Heck, if that poorest player is especially cash-poor, they might end up not being able to afford any of the shares even though they got first dibs. But for me, that's much more acceptable - more of a consequence of bad play on their part, or successful maneuvering on the part of their opponents.