While neither a narrow gauge nor a 2-6-6-2, Arnold made a small 0-8-8-0 Mallet tender tank loco in N scale.
For inquiring minds:
A very prolific model release, if you’re looking to acquire one of the out of production Arnold Gt 2x4/4 0-8-8-0 saddle tank articulated locomotives, you will find a complete list of releases and years of production below.
Arnold Stock Number 2256 Deutsche Reichsbahn Gesellschaft (DRG Gv Bay) Gt 2x4/4 Blue 1923 Seddin Exhibition Color Scheme with red wheels and road number 5773 that was produced from 1999 – 2002.

Arnold Stock Number 2257 Deutsche Reichsbahn Gesellschaft (DRG) BR 96 Black with red wheels and road number 96 016 that was produced from 1996 – 2002.

Arnold Stock Number 2264 Deutsche Reichsbahn Gesellschaft (DRG) BR 96 Black with red wheels and road number 96 020 that was produced from 1995 – 1996.

Arnold Stock Number 2275 Deutsche Reichsbahn Gesellschaft (DRG) BR 96 Black with red wheels and road number 96 016 that was produced from 1981 – 1992.

Arnold Stock Number 2276 Königlich Bayerische Staatsbahn Royal Bavarian State Railways (K.Bay.Sts.B.) Gt 2x4/4 Green with red wheels and road number 5751 that was produced from 1982 – 1991.

Arnold Stock Number 2277 Königlich Bayerische Staatsbahn Royal Bavarian State Railways (K.Bay.Sts.B.) Gt 2x4/4 Yellow 1926 Munich Exhibition Color Scheme with red wheels and road number 5751 that was produced from 1982 – 2002.

Arnold Stock Number 2278 Deutsche Reichsbahn Gesellschaft (DRG) BR 96 Black with red wheels and road number 96 001 that was produced from 1983 – 1985.

Arnold Stock Number 2282 Deutsche Reichsbahn Gesellschaft (DRG Gv Bay) Gt 2x4/4 Blue 1925 Paris Exhibition Color Scheme with red wheels and road number 5773 that was produced from 1999 – 2002.

Arnold Stock Number 2288 Deutsche Reichsbahn Gesellschaft (DRG Gv Bay) Gt 2x4/4 with green cylinders, saddle tanks, and wheels and road number 5772 that was produced from 1992 – 2002.

Arnold Stock Number 2289 Deutsche Reichsbahn Gesellschaft (DRG British – US Zone) BR 96 Black with red wheels and road number 96 022 that was produced from 1992 – 1998.

Arnold Stock Number 2705 Deutsche Reichsbahn Gesellschaft (DRG) BR 96 Limited Edition 24 Karat Gold Plated Unpowered Static Display Model with red wheels and no road number that was produced in 1982.

Hornby Arnold Stock Number HN2016 Deutsche Reichsbahn Gesellschaft (DRG) BR 96 Black with red wheels and road number 96 001 that was produced from 2009 - ???
