llSetForce train car

im making a subway car that will work with what SLRR uses but im not sure if what i did wrong is build related or script related, it works purfectly except for when going around turns it will topple over and i cant seam to fix it >.>

here is what i have for script so far
its nothing much just a simple off/on force touch would seting a bouyancy help at all?

integer on;
default
{
    state_entry()
    {
        on = FALSE;
    }

    touch_start(integer total_number)
    {
        if(llDetectedKey(0) == llGetOwner())
        { 
            if(on == FALSE)
            {
                llSetForce(llGetMass()*<8.1,0,0>,TRUE);
                llWhisper(0,"on");
                on = TRUE;
            }
            else if(on == TRUE)
            {
                llSetForce(<0,0,0>,TRUE);
                llWhisper(0,"off");
                on = FALSE;
            }
        }
    }
}

have you tried locking the tilt on x/y?
llSetStatus( STATUS_ROTATE_X | STATUS_ROTATE_Y, FALSE);