This really nice 3-rail AC HO gauge loco by Marklin is 3611 a K.W.St.E. Class C steam locomotive in grey livery. It has running number 2007. 

The steam locomotives of the Württemberg Class C were built for the Royal Württemberg State Railways in the early 20th century and were Pacifics designed for hauling express trains. They were the smallest state railway Länderbahn locomotives with a 4-6-2 wheel arrangement.

The 41 C class locomotives were built between 1909 and 1921 by Maschinenfabrik Esslingen. They were designed from the outset for the steep gradients in Württemberg and therefore had smaller driving wheels with only a 1,800 mm diameter. Despite being the smallest engines with a Pacific configuration, they were economical, yet at the same time very powerful.

The locomotives were known as "the beautiful Württemberg lass" (die schöne Württembergerin)

After the First World War, three engines were given to France where they became the État class 231 and one to Poland where it became the sole example of the Om101 class. The remaining 37 were taken over by the Deutsche Reichsbahn as DRG Class 18.1, where they were given the running numbers 18 101–137. They were even taken over by the Deutsche Bundesbahn, but retired by 1955. The last two examples were numbers 18 133 and 18 136, which were stabled in Ulm during their final years.

The three locomotives surrendered to France, 2021, 2026 and 2027, were allocated to the Chemins de Fer de l'État, where they were numbered 231–997 to 231–999. They were used until 1937, when they were placed into store for use as steam generators. The SNCF allocated them the numbers 3–231.A.997 to 999, but the locomotives never carried these numbers. The three locomotives were destroyed in an air raid on 13 May 1944

The loco is in good condition as you can see in the pictures, We are unable to test it as we do not have a three rail AC test rig or track, so we have not tested it but can see no reason why it would not run. It is sold in untested condition, this means that it may not run, and as such it is not being sold in running condition. However a complete external visual inspection has shown no reason why this loco would not run and the overall condition is very good. It could benefit from a little tidying of the pipes/rails on one side of the loco - see pictures. The box is in good if slightly shelf worn condition. The inner plastic sleeve is yellowing and starting to go brittle, common with Marklin products of this era

Overall this is a really nice scale locomotive and would look fantastic on a layout.

Do please have a look at my other listings, I typically have various HO and OO locos on offer as well as an extensive range of imported N gauge locos and other items. 

Note: I do recommend all locos are cleaned & serviced, as they have some age to them even if the quality looks at the highest level (fairly standard advice for all locos!).