Modern model diesel locomotives usually have all wheel pickup. Unfortunately state of the art Steam Locomotives seem to overlook the advantages of all wheel pickup and rely on split frames and often ignore the tender as a source of power pickup. On other occasions the electrical pickup is from only one side of the tender.
Simple solution is at hand. Phosphor bronze (PB) sheet is available from Walthers in various thicknesses. The thin stuff works best. You may find a local source of Phosphor bronze wire which can be made to work as well.
The photos show a typical installation on derlin trucks with metal wheels on insulated axles.
Cut a piece of PB the width of the tire tread and long enough to bridge from the top of one wheel to the other. Drill a small hole in the middle of the PB to allow a piece of flexible multi-strand flexible wire to pass through. Strip about 3/8 inch of the wire and flux the wire and the PB at the hole. Tin the wire and the PB with solder and re-drill the hole. Insert the wire extending through the hole and touch it with the iron.
Drill a matching hole through the truck bolster between the wheels and insert the wire. Pull the wire down until the PB bottoms out on top of the bolster and fix it there with Black Tire ACC. Bend the wire over on the bottom of the bolster and fix it at that location. Now you have a set of contacts out of sight behind the top of the side frame that act as springs on top of the wheels. To reduce drag carefully lift and adjust the tension of the PB.
Drill a 1/8" hole about 1/3 of the way in from the end of the body bolster on each side and pass the insulated length of wire inside the tender body. Wrap the extra wire around a screw driver inside the tender body to make a flexible coil to keep the wire out of the way. Add these wires to you pick up grid inside the tender.
Similar installations can be made on brass locomotives and trucks. Just make note to insulate the mounting of the PB wiper with an intermediate layer of styrene.
If the installation allows you may find having the wiper ride on the back side of the wheel an advantage. This avoids the dirt that will eventually build up on the wheel tread.
Try it, you will like it!