i did a conversion of the old type 47 chassis and didn't solder wires to the bogies cos i thought the drag would restrict the bogie movement, but you end up with a chassis block that is live to the bottom brush on the motor all be it on the grey or orange lead, but you may end up with that anyway. what i did,was on top of the actual bogie on the outer end of the gear tower i soldered a brass pad the full width of the bogie middle the gears and axles fit in. i then drilled the chassis dead centre above where this 'pad' now was and fitted a sprung plunger like the type used for current pick up on O gauge models usually but they may have been a 4mm version?? ( it was years ago ). the other feed from the inner of the towers was isolated from the brush it usually feeds. then you basically join the 2 plungers to red or black, the two from the inside of the towers ( little nuts on top of the black plastic top to the chassis to the other decoder feed ( red or black). the actual spring retainers for the brushes have the grey and orange wires soldered to them as necessary. all tickety boo. what i did find ( may have been the old DN r DZ 14 whatever i used ) but the speed control was poor ( i didn't try or use speed tables back then ) it would take a bit to get moving at a walk,a couple of clicks later it would go to about 1/3rd to half speed!! nothing in between. i decided at the time that the mechanism was very draggy with friction, or the farish motor just didn't like the output from the decoder. i actually removed the decoder and just wired it back to run on DC with all the chassis and pickup mods still in place. it would be a simple task to try a 'new' decoder in it and see if things have changed.