The 18136 and 18129 are identical. The difference is that the 18129 came in a set with a powered A unit, item #18128, and the 18136 was available as a separate sale item a year later. The set with the no. 18128 powered A unit and no. 18129 B unit was numbered 18130, and was made in 1996. The 18136 B unit was made in 1997.
I assume when you say "autocouplers" you mean remote controlled "electrocouplers." That's the term Lionel uses for them. Yes, Lionel said electocouplers in their ad, but despite the ad, neither the 18136 nor the 18129 was equipped with electrocouplers. I have see a photo of the bottom of a 18136 and didn't see any electrocouplers, and there has been other commentary about this. None of the independent ads for the 18136 say they have electrocouplers, either.
However, Lionel made a separate sale non-powered A unit (item no. 18134) to go with the set. These non-powered so-called dummy A units are normally put behind the B unit, with the front of the A unit facing the rear, and connected to the cars. This dummy A unit is equipped with an electrocoupler, which is meant to be used to connect and disconnect the cars the engines are pulling. So if you want an electrocoupler with the set, you need to add the no. 18134 dummy A unit. This arrangement was common in a number of similar F3 sets during this time.
Several years later, Lionel started adding electrocouplers to the rear of these B units as well. Sets were often offered as powered A and nonpowered B units, and an electrocoupler on the B unit meant it was no longer necessary to buy a separate dummy A unit to get one.