I use very fine lacquered wire that I stripped off an old transformer. I braid six strands together to get good current handling capacity. The resulting wire is well insulated yet very flexible. You have to prep the ends real well to burn the lacquer insulation off so you can solder it. I have quite a few locomotives with Vanderbilt tenders where I can not use the phosphor bronze wiper to transmit power because you can't hide them under the curved sides of the tender tank very well. On these I use soldered wire to the truck frames. I have no problems with them at all.
What if I were to add a little tention on the ends of the leaf springs that ride on top of the wheel contact tabs? Maybe just a dab of glue along the top to add wieght.
...Also, What if I were to cut out the center axle groove of the axle contact so the contact would be forced to ride on the front and rear axle of the truck. This would remove the possibilty of the contact pivoting off the end axles.
On the Concor S2/GS-4 tender, you need to have the leaf spring intact. Without downward pressure on the pickups, there is nothing there to force a contact between the wiper and the wheels. The same pickup is used on the 4-6-4 tender and I really have come to hate it. Although the S2/GS-4 version is executed a little better. On the Hudson tenders, I have resorted to filing all the slots in the axles with mothers mag wheel polish, weighting the tender down as much as I could and dragging the tender behind another loco on a test loop for quite a few hours allowing the contacts to polish themselves and wear in so that all wheels contact evenly, or at least as best as I could. The wipers that ConCor uses for both the truck and the leaf spring leave a lot to be desired. The Kato version of the S2 used better materials that didn't corrode as easy it seems. If you feel real ambitious, the best solution may be to order up a set of Kato GS4 tender trucks and stick under it. They have needle point pickup and the center axle is sprung so that at least 5 wheels per truck can maintain contact with the track at all times.
I have found that I can bend the ends of the leaf spring strips a little to create more tension. Also Atlas conducto-lube seems to help my pick-up problems a lot. Very clean wheels and a drop of the lube on the wheel contact flanges and the spring surfaces gets them running smooth in my experience.
