Although uberz91's solution should work fine, I came up with something different, because the use of SPD is restricted in many companys. My solution is a bit fiddly and may not work for everyone, but there is no coding or SPD required. :)
I created several content types and added a custom approval workflow to every content type. For every available value in the triggering column (car, plane, train), a content type is created. Each content type workflow triggers a different group of users.
One problem of this idea: The content types have to be chosen through the dropdown menu in the list. There has to be a default content type, which is chosen by the click on "Add a new item". I avoided this by displaying my list in a web part with a toolbar. Next to the web part I created links, which launch the different content type templates (I found this tutorial useful). Essentially I am bringing forward the selection between car, plane, train etc.. Instead of being a choice within the template, the choice is made before the template is opened.
Another problem: As I needed the car-plane-train-column as a meta tag inside the list and you can not refer to content types in calculation columns, I just fill this column with a conditional default value according to the content type. Every content type has its own template/form with a different default value for that column/meta tag. The text field can be disabled or hidden with Infopath, I decided to show the disabled field to remind the user that a value was already chosen.
A global approval status can then be set, by updating the Content Approval with the Approval Workflow (as discussed here).
Again, it's fiddly and far from perfect, but maybe this may help others with restrictions on SharePoint Designer, too. :)