Great photos. So it appears that in this style passenger car, MTH does the AC-to-DC conversion as one would expect on the lower board (with DCS choke...albeit apparently an after-thought since it's not on the board). Then, DC is passed to the upper board via the pair of spring contacts. That is, the upper board does not have diodes installed so it is operating on DC. While mission-accomplished here, the DC interface between lower and upper might be used to advantage in a different project.