[1868Wyoming] train cross-buy is not working about 18xx HOT 2 CLOSED

From experimenting in hotseat a little more, all trains except 4+3 trains can be cross-bought from other companies. Not sure what would cause that but it’s at least a lead for when I have time to properly debug this.

from 18xx.

• the base code determined that both the 4 and 4+3 variants should be available for other corporations to buy (other code made sure the UI only presented the 4+3, which is how the train was bought initially by LNP)
• FE&MV has $325, more than the face value of a 4-train ($300) but less than the 4+3's face value ($360)
• the 1868 Wyoming code assumes that if two variants are available, we're buying from the depot
• when buying from the depot, we can only buy a train we can afford or the cheapest, so the 4+3 gets filtered out

The fix in #10793 is to simply return the single active variant if the train is in fact owned by a company instead of the depot.

from 18xx.