One thing all my shareholding train games have in common is that once you buy a share, it's yours for keeps. You can't sell off your holdings in one company for some quick cash to float a new one, as you can in the 18xx. Neither can you "buy low sell high", divesting yourself of a company when it's at its apex, deftly avoiding the inevitable fall.
This is something that used to irritate one of our playtesters to no end. Every time a playtest was concluded, every time we asked for feedback, she always said that she wished there was a mechanism by which she could sell shares. Partially this was because she would sometimes find herself in situations where she would very much like to dump one company in order to start another, partially this was because she didn't want to see her share values plummet, and partially this was a thematic bugbear: real stocks don't work that way, people buy and sell them all the time.
I even tried once or twice to design games that allowed for the selling of shares, but every time I was dissatisfied with the results, and ended up reverting back to my standard. Mind you, nowhere near as dissatisfied as that playtester was once I had done so: Last week I could sell shares, and now I can't? You're making the game worse! And maybe I was.
But especially in Trans-Siberian Railroad, Iberian Gauge, and The Soo Line, I was interested in ownership of a share conferring a certain degree of liability. In Trans-Siberian, companies that underperform can (and usually are) Nationalized, a nasty bit of business that renders every share worthless. In Iberian Gauge and The Soo Line, companies can see their share values drop dramatically within a single round - with the number of boxes dropped equal to the number of shares in player hands. That is, the more people buy into the company, the greater the liability if its expansion stalls, and the more you personally buy into that company, the more it's going to hurt your position in the game.
Share ownership as liability is nothing new in the train game space, of course. In many of the 18xx, being President of a company (having the most shares) means you're on the hook for making sure that the company has enough money to buy a train should the old one rust. A forced train purchase might not result in bankruptcy, but it's almost always going to result in fatally damaging your chances of winning. And the fear of being stuck in that position makes many players very timid about buying a second share in someone else's company. I'm not actually sure if I like that particular liability, but whether that has to do with the way it disincentivizes passive investment strategies, or if it's because of that time Kenny dumped a penniless piece of garbage in my lap, I will leave up to you, gentle reader.
Whichever it might be, being able to sell shares when you can see trouble coming down the line, or even as a way to cause that trouble for other players, isn't something I'm particularly interested in exploring as a designer. I'd much rather players be forced to live with the consequences of their decisions; that makes each stock purchase much more meaningful and fraught.I actually leaned into this a bit with London and Northwestern and The Soo Line, both of which allow you to buy only one share per round. That gives players far less wiggle room than games where you have more purchasing opportunities. Not buying a share is a terrible idea - falling behind on the shares race will sink your position - but so is buying the wrong share.