How to Encourage Balanced Portfolios?

A basic aim of Britain Under Steam is to encourage players to invest in each others' companies without restricting which companies are available to start. This will combine what I see as the best of two different strands of 18xx games. 1825 and its descendants force cross-investment by making the companies available in a set order, so that players have to buy shares in those that are currently available. 1830 and its descendants let players start any company, with the result that they tend to focus on buying shares in the one(s) they have started themselves. The question is, how do I design the start of the game to encourage cross-investment?

One promising approach is to be fairly conventional, by requiring 60% of a company's shares to be sold before it floats, limiting each player's holdings to 60% of a given company, and giving players enough starting cash to buy shares equal to 100% of a cheap company. This means that a player can start her own company and have enough cash left over to buy shares in other players' companies.

I was wondering whether I could encourage more cross-investment if I companies floated when 40% of their shares were floated. This would leave more player cash available to invest in other companies. This idea does immediately run into a flaw: if a player has enough cash to buy 80% of a company, she could start two companies instead of investing in other players' companies. I have worked out rules to balance the options of starting one company or two (including the potential for asset stripping), but these don't address the basic aim I am considering here.

In 1830-style games, of course, directors are forbidden from selling the directors certificate, and companies without a train must buy one. This combination ensures there is a risk involved in starting a second company. Britain under Steam doesn't have these restrictions, which removes much of the risk. This is intentional, as it also removes the risk of cross-investment, but perversely it also discourages cross-investment if players have the option of floating a second or third company instead.

I could add an administrative fee for starting a company, which would perhaps encourage players to buy existing shares instead. However, this might have the unintended consequence of making it too unprofitable to start a company at all, which would hardly be the desired behaviour!

A variant on that idea would be charge no administrative fee for a player's first company, introduce a fee for their second, and a higher fee still for their third. The cost could be based on the number of directorships that the player currently holds, or the number of companies they have floated since the start of the game.

Another take on this variant would be to deal the companies in order at the start of the game, then make the one at the head of the queue free to start and ones further along cost a small fee. This second version would restrict players more than I would like.

A different approach would be to limit players to holding a single directorship in the first phase of the game. As soon as phase three begins, this restriction would be lifted.

It's worth noting that in the advanced game, I'm thinking of adding auctions for the right to start a new company. A player can nominate a company and a par price; whoever then bids the most wins the right to buy the directors' certificate at that par price. I'd have to work out how this would sit with any additional fee for starting a company.

At one point I thought about charging a administrative fee every new company, but giving the owners of some private railways a fee waiver. I haven't revisited this idea until now; perhaps it is worth some reconsideration?