What I meant about a lose wire could be if you open up the locomotive (probably shown in the manual how to take the top off) and then check that all the different wires are in place and solidly connected to the places they should be soldered to. It could be a simple thing like the wire going from the pickup shoe (where it gets the power) is loose or broken, and then find the correct place to put it (and solder it back on). It's a long time since I have messed with decoders and stuff. There might be wires soldered to the decoder (there may even be more then one decoder if a second, sound decoder is on top of the base decoder... if I remember correctly?). The decoders themselves have multi pin connectors, like stuff in computers and it can be checked if they are seated (pushed down) correctly. Maybe someone has taken it off and put it back in incorrectly?

Because of the low price there is the possibility that it's a return from someone else who bought it and couldn't get it to work.

Also sometimes some wires may be connected via plugs or connectors and you can check if the wires are firmly connected.

If sending back to Marklin do not open and do work which is obvious as Marklin will not repair. I know I keep saying but why are you not returning for a refund and then buy something from a Marklin dealer.