c++ sample test for Train Composition

I've modified the code as below for a sample test as given in http://ift.tt/2sYBx46

"A TrainComposition is built by attaching and detaching wagons from the left and the right sides. For example, if we start by attaching wagon 7 from the left followed by attaching wagon 13, again from the left, we get a composition of two wagons (13 and 7 from left to right). Now the first wagon that can be detached from the right is 7 and the first that can be detached from the left is 13. Implement a TrainComposition that models this problem."

#include <stdexcept>
#include <iostream>
#include <vector>

class TrainComposition
{
    //std::vector<int> wagons;

    public:

    std::vector<int> wagons;

    void attachWagonFromLeft(int wagonId)
    {
        wagons.insert(wagons.begin(), wagonId);
    }

    void attachWagonFromRight(int wagonId)
    {
        wagons.push_back(wagonId);
    }

    int detachWagonFromLeft()
    {
        int ret = wagons[0];
        wagons.erase(wagons.begin());
        return ret;
    }

    int detachWagonFromRight()
    {
        int ret = wagons[(int)(wagons.size()) - 1];
        wagons.pop_back();
        return ret;
    }
};

#ifndef RunTests
int main()
{
    TrainComposition tree;
    tree.attachWagonFromLeft(7);
    tree.attachWagonFromLeft(13);
    std::cout << tree.detachWagonFromRight() << "\n"; // 7 
    std::cout << tree.detachWagonFromLeft() << "\n";; // 13
    return 0;
}
#endif

The test result has one fail as Performance test with a large number of wagons: Time limit exceeded

Please help to fix this fail...