18xx Companies able to buy other companies, what will be the impact
I'd love to see a proper set of Merger &amp; Acquisitions rules for 18xx.

I suspect I would start with something like this:


Companies may buy each other's shares with company cash.
Companies may not buy more than 20% of another company without making a takeover bid.
To make a takeover bid, a company must have enough cash to pay current market-price for all the shares it doesn't hold.  It first buys all shares of the target company that are not owned by players or other companies.  If that gets them >50%, then the takeover is successful.
If it doesn't get them >50%, then they offer current market price to the other shareholders.  If enough accept to get to >50%, then success, otherwise, they can raise the offer until they succeed, run out of cash, or give up.
Any remaining shares are then forcibly bought at the highest price reached in the takeover auction.
Once the company has been taken over, the new company has then eaten it.  It can issue the share certificates of the takenover company as additional shares, diluting its own shareholdings.


That's a hostile takeover system.

A merger would go more like this:


If two companies each own 20% of each other and both presidents want them to merge, then they should announce this.  They need >50% of the shares to vote in favour.  Shares owned by each other automatically vote yes.
There is now one, bigger company with all the station markers and trains.  If there are rules on the maximum number of trains a company can own, then this company can own twice as many.  Share certificates now represent half as large a percentage as previously.
The two Presidents' shares are combined into a single certificate, so that shareholding isn't diluted.  The new President will have to exchange some of his previous shares to get the shareholding right (ie Pres share is normally 20%, the President's certificate they had before is now only 10%, so they will have to hand over two other certificates to balance it out: if they don't have that many, then they can buy them from the company; it certainly owns at least 20% of itself).  If both of the Presidents of the pre-merger companies have the same shareholding in newco, then they have to agree when they announce the merger.
Any remaining shares that remain with the company after forced purchases by the new President and making change to the old President are now available to be bought on the open market.
The new share price is the average of the share prices of the two old companies.