It's the soundtrack of a drama.
Shueisha released 2 CD Books in 1996/1997 集英社ＣＤブック
快傑 蒸気探偵団 File 1 (19/07/1996) & File 2 (24/10/1997).
Each of these contains a drama CD and a book (illustrations, interviews for File 1 and notebook for File 2).
CD drama 1/File 1: SCD-136
CD drama 2/File 2: SCD-139
Can see them here:
http://page7.auctions.yahoo.co.jp/jp/auction/g88415164
A soundtrack for each "File" has been released later on:
- 21/08/1996, AYCM-563: the OST for File 1
http://www.amazon.co.jp/%E5%BF%AB%E5...81%B5%E5%9B%A3
- 03/12/1997, KICA-381: the OST for File 2. This CD. You'll notice that " File 2" is not printed on it...
I would be you, I'd change category to "Radio/Drama" and rename this CD including its subtitle "The Duel! Narutaki vs Little Thief Lu-Bread!!" because the first drama OST is named "Original Soundtrack and Inspired from STEAM DETECTIVES File 1". To avoid confusion...
The 2 drama CD (included in CD Book) are falling under "publication" category.
And if we want to be complete, there's also a drama CD for the Tv series, "Blue" (KICA-464, 23/07/1999)