How to do pre-reading derivatives take-two

How to do a pre-reading derivative handout:
train, trail, tractor, intractable < ___________________

Definition: not easily controlled or directed; not docile or manageable; stubborn; obstinate:an intractable disposition.

"intractable" Etymology online doesn't give a word in vocabulary list. So try tractor: 

tractor (n.) 1856, "something that pulls," from Modern Latin tractor "that which draws," agent noun from past participle stem of Latin trahere "to pull, draw" (see tract (n.1))

OR, since the dictionary says for "intractable" says "see tractable" go to that definition in Dictionary.com and you will find in word origins:

from Latin tractābilis , from tractāre to manage, from trahere to draw.

Use both dictionary.com and etymology online to get back to the Latin word in the vocabulary list.