  The coherent superposition of two-atomic levels induced by coherent
population trapping is employed in a standard $\Lambda$ type scheme to form a
tripod-like system. A weak probe pulse scanning across the system is shown to
experience a crossover from absorption to transparent and then to
amplification. Consequently the group velocity of the probe pulse can be
controlled to propagate either as a subluminal, a standard, a superluminal or
even a negative speed. It is shown that the propagation behavior of the light
field is entirely determined and controlled by the initial states of the
coherent superposition.
