Node:Subsidiaries, Next:Move Error Popups, Previous:New Vehicle Graphics, Up:The Patches
Normally, there is little incentive to buy shares of another company, except for buying it out entirely. Even if you own 75% of the shares, you get none of the profit, you get no control over the company and it's as if you didn't own any shares at all.
Using the new subsidiary management however, you can manage a company that you own 75% of. Once you own 75% of a company, you can open its player info window, and the usual "View HQ" button will become a "Manage" button instead. If you click this button, you temporarily switch companies, and can now build new services, remove services or do anything else you want.
When you're done managing the subsidiary, you can open your own player window, and click the "Manage" button there to return control to your own company.
Note that after you manage another company, the computer AI will take over the management of that company, and it may do with it what it wants as well. This means that you should try not to build confusing (to the AI) train routes or services, because it may decide that they are unprofitable and remove them again.