The Collaborative International Dictionary of English v.0.48:

Trainable \Train"a*ble\, a.
   Capable of being trained or educated; as, boys trainable to
   virtue. --Richardson.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "trainable":
   aimable, aimed, apt, bright, clever, directed, directional,
   directive, dirigible, docile, educable, facile, formable, guidable,
   impressionable, instructable, intelligent, leadable, malleable,
   moldable, motivated, on the mark, on the nose, plastic, pliable,
   pointable, quick, ready, receptive, ripe for instruction,
   schoolable, steerable, susceptible, teachable,
   thirsty for knowledge, well-aimed, well-directed, willing