Dropper wires, for many years I have used the same method particularly on plastic track. The dropper is a short bit of domestic lighting cable stripped of insulation. Drill a just big enough hole down through the base board about 1/16in (1.5mm) from the rail. This can be the opposite to the viewing side. Now take your bit of thick wire and bend a upside 'L' shape the leg being long enough to reach from the hole under the rail. Thread through hole and turn end under rail. If it slips down put a bit of card/wood between board and leg to support. Touch of flux, in and out quick with a hot iron, good joint and no damage to plastic. If the 'L' lag was over length it can now be cut off. Once ballast and weathering complete will be invisible. It is now easy to join up the other wiring from the under side only as normal 'layout' flexible wire can be twisted to the leg and will hold itself in place for soldering.
Hope this helps
Regards
Allen Morgan
Hope this helps
Regards
Allen Morgan