The potential of fast in-line saturable absorbers is investigated for multiwavelength soliton systems. Numerical simulations demonstrate that both synchronous and asynchronous 4 x 10 Gb/s transmissions are possible with this technique. Limitations by finite absorber dynamics are also analyzed. Q factors close to 10 are obtained for a 4 x 10 Gb/s transmision with constant dispersion, provided that the absorber recovery time does not exceed 10 ps, The results demonstrate the capability of saturable absorbers to enhance performance in passive wavelength-division-multiplexed (WDM) soliton systems.