At the rear, you should give up the battery box to mount a coupler? This would distort the typical aspect of the railcar, and would remind me the Jouef model of the 1960s in an annoying way. 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 draw bar, passing under the battery box, doesn’t come to stop behind it.
We see that the old plastic draw bar 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 draw bar, to benefit from its greater thickness. Pin 3 is used to maintain alignment. It will have to be carefully leveled 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 coupler to get the shortest possible coupling.
Warning: as I’m a little lazy, I haven’t shown all the old draw bar 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 draw bar is made in a dia. 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 draw bar, 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 millimeter. The differences with the previous view are explained by the fact that the dimensions are not taken at the same locations.