Lionel made two versions of the Wabash geep.
The Lionel 2339 Wabash GP-7 was cataloged in 1957 only. It was an O gauge engine and was available in sets or by itself.
The 2337 GP-7 was made in 1958, and cataloged as an 0-27 loco. 0-27 locos mostly only came in sets, and 0-27 sets came with track and transformer.
The two engines are identical except for the number on the hood. Neither version is harder to find, but the 2339 is slightly more desirable than the 2337.