The S-2 Steam Turbine Type 6-8-6 Locomotive No. 681 was based on a prototype built by the Pennsylvania Railroad. This engine was deemed inefficient by this railroad and they only made one of them. However, in the "euphoria" of post-war toy train production, Lionel made thousands of them!

This locomotive replaced the No. 671 in 1950. The main difference was the addition of magnetraction that necessitated the removal of the nickel rims on the drive wheels. Produced from 1950-51 and in 1953, this locomotive usually came with either No. 2671 or 2046WX (2046W Streamlined) tenders that had a whistle and came with bar-end trucks with one magnetic coupler.

This locomotive came with a smoke unit, headlight, and was controlled by a three position directional unit whose control lever is adjustable from just in front of the cab. There is a Keystone decal on the front that is numbered "6200". This locomotive is equivalent to the 027 No. 2020 and this, and other versions of locomotive that were produced by Lionel for O and O27 gauge sets run best on the larger diameter O gauge track.

There are two variations:

Variation A: SILVER rubber-stamped number on cab.

Variation B: WHITE heat-stamped number on cab.