Speed of the TRAX is given as 16 m/s so let say it is given as
now our speed towards the front end is given by 5 m/s so this is the relative speed of us with respect to TRAX
let say this speed is given as
now we need to find the speed with respect to someone standing outside the TRAX
so here we need to find the net speed in ground frame and hence we can use the formula of relative speed
so someone outside the TRAX will see our speed as 21 m/s