The instructions are the same as the 4228 Rheingold set, as far as couplers go, so you may be able to find more photos of that to help. One thing in the manual for the 2681 set that isn't clear from the drawings is this: With the current-conducting couplers, the metal strip faces up.

Also, for the current-conducting couplers, install the coupler into one car, set the car on the track, then place the second car on the track and push them towards each other. It is usually necessary to guide the coupler into the socket on the 2nd car, and to place a finger or other object underneath the coupler as it slides in to keep it from flexing as it goes in. Make sure the coupler "clicks" into place on the 2nd car. (Do not ask me how I know to check for this ).

Good luck

Hi
You can also take a look at the manual for the Orient Express 42795
https://www.maerklin.de/en/products/details/article/42795/