The S-2 Steam Turbine Type 6-8-6 Locomotive No. 671 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 did not have magnetraction and the tenders came with staple-end trucks with one electronic coupler. This, and other versions of locomotive, were produced by Lionel for O and O27 gauge sets, but it runs best on the larger diameter O gauge track.

Over the five years that this engine was produced (1946-1949) there would be six variations:

Variation A: Directional Unit and whistle in tender are connected by jacks to the motor that was labeled "ATOMIC MOTOR" in the locomotive. A double worm drive provides the power to the drive axles. In the front of the locomotive is a RED Keystone that has GOLD lettering, and on the sides, it is equipped with shinny nickel rims on the drive wheels, and it came with a No. 671W tender that has WHITE rubber-stamped "LIONEL LINES" lettering on the sides and has grab railings on the sides and the deck. (1946)

Variation B: Identical to (A) except it has a BLACK Keystone on the front with WHITE number "6200" that was used on the Pennsylvania prototype. This is the first production of this locomotive. (1946)

Variation C: Has an on-board direction unit whose lever protrudes though the top of the boiler and a single worm drive motor that is labeled "LIONEL PRECISION MOTOR" that no longer has the jacks. The Keystone is RED on the front of the locomotive and the No. 671W tender has a whistle with all of the railings that were on the previous versions. (1947)

Variation D: Similar to (C), but with blackened steel rims on the drive wheels and came with a six-wheel truck No. 2671W tender that has SILVER rubber-stamped "PENNSYLVANIA" lettering on the sides. It is this tender that makes this variation the hardest to come by. In addition to the prototype lettering, it also featured RED backup lights. (1948)

Variation E: Similar to (D), but without the backup lights. (1948)

Variation F: Identical to (E), except there are no steel rims on the drivers, and the lettering on the tenders is almost always heat-stamped. (1949)