Sorry to have noticed this question so late..! :(
but here i give the explanation to ur doubt..which, i am sorry to say will mutilate ur beautiful thought process!! :-(
assassin had given correct explanation! !
if we imagine the train to be moving forward with a velocity v1 and the runner inside it moving with a velocity v2 in the forward direction w.r.t. the movement of the train, we might expect, from our knowledge of classical newtonian mechanics, based on Galilean transformations, we would conclude that the velocity of the man w.r.t. ground is v1 +v2
but that is not true.
infact the laws of classical mechanics are not true...
we need to change our views to the lorentz transformations..
and according to te lorentz transformations,
v=(v1+v2)[1+(v1v2/c2)]
this answer may be readily derived as...
if the runner covers a distance of dx in time dt, his ground relative speed is v=dx/dt
from lorentz transformations we may write,
dx/dt= (dx'+v1dt')(dt'+v1dx'/c2)
or, dx/dt= [(dx'/dt') + v1][1+(dx'/dt')(v1/c2)]
or, v=(v1+v2)(1+v1v2/c2)
noth that as a consequence of velocity addition law, if the runner is replaced by a light wave(v2=c), then automatically v=c thus satisfying the second principle of relativity, which states that light's speed is constant for all inertial frames of reference.! :)
this law also implies, no matter what values of v1 and v2 we choose (if less than c itself) the resultant velocity is always less than c!! :)