How to make sure I use specific class feature only when provided in c++?

Lets say I have class that use another class as feature.

class city{

    void travel(){
        // if rail road travel by rail

        // if no rail road travel by foot 
    }

private:
   RailRoad rails;
};

When I instantiate city

case 1 : I can have city with railroad.

case 2: I could also have city without railroad.

I thought was use RailRoad * rails and use nullptr as flag to determine the case. I was trying to avoid this because for this I have to do new and delete.

Is there any elegant way to achieve this?