There are 2 issues to solve.
1) Creating a source of lower track voltage other than by manually lowering the throttle handle. To that end, take a look at this OGR thread.
2) Triggering when to swap-in and swap-out the lower voltage. This is the classic occupancy detection problem with the list of usual-suspects - insulated-rail with relay, pressure plate (e.g., Lionel 153C), Infrared/optical detector (Lionel 153 IR), and so on. Details such as specifying if train run both directions (i.e., don't reduce voltage if entering grade going uphill) will narrow the choice.
