I recently got a PIKO expert Taurus lok. It was 2 rail with an 8 pin socket I looked at the parts list and the lok and noticed parts for an AC.
A
fter a couple of emails to PIKO USA and got some parts I now have a 2/3 rail lok.

I wired all my power pick-ups to a 4 point bread board terminal strip.
1. was power from center pickup
2 was power to decoder
3 was left track power (not at all connected to decoder)
4 was right track power

Now when I run 2 rail, I remove center pick up from lok and place a jumper between 2 and 3.
When I run 3 rail, I install center pick up and place one jumper between 1 and 2 and another jumper between 3 and 4. I did not need the 2nd jumper but it does help with getting more ground contacts.

If I would have found a tiny double throw switch, that could have helped.

Some of the Marklin/Trix loks are close to being this simple.

It´s very sad that only 47 did voted under one month.
But i´m surprised 34 % do welcome multi locomotive.
I did voted for yes.