The S-2 Steam Turbine Type 6-8-6 Locomotive No. 681 was based on the S-2 steam turbine 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 to 1951 and again in 1953. It was not produced in 1952 because of the shortage of magnetic material due to the Korean War. This locomotive usually came with either No. 2671W or 2046WX (2046W-50 Pennsylvania) streamlined tenders that had a whistle and came with bar-end trucks with one magnetic coupler. Note that the above pictured No. 6020W tender is normally found with Locomotive No. 2020 Variation D.

This locomotive came with a smoke unit, headlight, and was controlled by a three position directional unit whose control lever is adjustable from the front of the square Belpaire smoke box that was used by the Pennsylvania Railroad. There is a Pennsylvania Keystone decal on the front that is numbered "6200".

This locomotive is equivalent to the 027 No. 2020. This, and other versions of the S-2 locomotive were produced by Lionel for both O and O27 Gauge sets. However, because of length of the eight driving wheels these locomotives 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.