The three optional rules (all suggested by the designer) are detailed
below.

There's an initial auction. Not too weird, but if you're outbid you
don't just get the money back, but can increase the bid and at the
same time move it to another item. Stuff can go unbought.

No more than 5 shares per company in the market; can't own more than 6
shares; non-Director shares only cause a drop if 2+ are sold. Can't
sell in the first stock round. Go up one for being fully player owned.
Optional rule #1: priority is in order people pass, not clockwise from
the first to pass. Optional rule #2: if you sell 2+ shares in a
company that hasn't operated, you get paid as if it dropped 1 spot
after each share sold, rather then (say) sell three, get 3x current
price, it drops 3 spots.

Companies can own shares in other companies. Only the Director of a
given company can buy _other_ company's shares from it. When the first
final-phase train is bought, there's a "President's Choice" stock
round under that constraint, but then you can buy any share from a
company whether or not you are the Director.

Game ends by bank breaking, playing until the next end of a set of
ORs.

When you launch a company, you have a choice of par values depending
on which company it is, and the par value affects whether it floats at
40/50/60%. Incremental capitalisation.

Buy in privates at 50-150% face value; if the private is in the
market, at 50%. A company can issue/redeem its own shares and also buy
in or sell other companies' shares, but it can't buy a Director's
share to start another company. It can also buy shares from its own
Director, but (optional rule #3) may not do so if this would cause a
change of Directorship.

One or two tile lays, but if two, one of them must be yellow and they
can't both be the same hex. There are "towns" as well as cities (large
stations) and villages (dits). Towns upgrade to cities in brown;
they're shown on the 18xx.games map as dits but with little
up-arrows. Some hexes have mine tokens, you take them when you build
on them and get $10 to the treasury for each token every OR iff you
had any train income.

Big gotcha; you can't build "into" plain track tiles when upgrading
them, you have to have had a route to the tile before the new track
was laid.

Routes must have at least one end be a city or offboard location. I
think. Maybe both ends, I'm not sure what to make of 5.8.8.1 if you
didn't use your full city quota.

Towns and villages give revenue but don't count against train
range. nD trains count n cities and offboards, then double them, but
ignore towns and villages. E trains have unlimited range and count
income normally. There's an East-West route rule.

Discarded trains are half price. Trains from other companies go for
$1 minimum with no maximum. You only need have a train if you have any
possible route. There's one of those mechanics where later in the game
trains in the next tier become available as soon as one in the current
tier is sold, not all.

If a company would be a Director of another, it absorbs it; remaining
shares the absorbed company had are sold, the absorbed company's
treasury divided between the shareholders; the absorbing company gets
up to 2 station tokens, the trains, the mine tokens, privates, and a
permanent +1 train limit.

There's an acquisition/liquidation range on the market. Companies here
are sold off to solvent companies. A company can't bid if it has the
same Director as the insolvent company.