At the rear, you should give up the battery box to mount a coupling? This would distort the typical aspect of the railcar, and would remind me the Jouef model of the 1960s annoyingly. Something must be done!
This is the rear of the Picasso:
Photo © Auzéau 2008.
Picture shamefully forged by me.
The most important is that the future drawbar, passing under the battery box, doesn’t come to stop behind it.
We see that the old plastic drawbar will be cut at the level of its NEM box. The fixing screw 4 will pass through a hole tapped in the guide pin of the plastic drawbar, to benefit from its greater thickness. Pin 3 is used to maintain alignment. It will have to be carefully levelled so as not to rub under the body. Finally, pins 5 (from a 1/10’ pitch connecting bar for PCB) will receive a specially prepared Roco or Fleischmann coupling to get the shortest possible coupling.
Warning: as I’m a little lazy, I haven’t shown all the old drawbar details, especially the return spring fastener. Do not deduct that it should be identical to the above drawing…
Here is an exploded view of the assembly. The main part is a 0.5 mm thick and 5 mm wide strip of brass. For the holes, see the figure below. The M1.6 tapping in the plastic drawbar is made in a diam. 1.3 mm hole. The pins’ height is about 5 mm; they’ll have in any case to be adjusted after assembly.
Here is another dimensioned view of the drawbar, this time flat, before folding. It is always delicate to dimension a fold. I estimated the folding inside radius to be 0.5 mm. But don’t take the given values literally. The accuracy doesn’t have to be 1/10th of a millimetre. The differences with the previous view are explained by the fact that the dimensions are not taken at the same locations.