This really nice 3-rail AC HO gauge loco by Marklin is 3791 a DR BR 03.10 streamlined steam locomotive in grey livery. It has running number 03 1056. This is a very striking locomotive.  

The locomotives of the series 03.10 were standard express train locomotives of the Deutsche Reichsbahn , which were built from 1939 to 1941. The 03.10 series was a further development of the 03 series , comparable to the 01.10 series locomotives , which emerged from the 01 series . The production of 140 units was planned. However, only 60 locomotives were completed due to the start of World War II and the shift in production to "war essential" goods.

In the 1930s, the subject of speed played a major role due to the competition from cars and also from streamlined railcars. Tests in the wind tunnel showed that higher speeds could also be achieved for steam locomotives with a streamlined casing. Measurements showed that the fairing reduced drag by 50%. So in 1936 it was decided to build fully enclosed locomotives with triple engines as the 01.10 and 03.10 series.

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. 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!).