Originally Posted by bike_tom

Happy to oblige. At first I thought posting the pics you are asking for would have to wait till the next time I am out that way. Then I remembered that I was recording video the whole time I was on the Elm Brook Trail. So, I was able to create a couple of gifs from that video, showing the entry and exit points of the detour.
This shows going southbound on South Rd just after leaving Depot Park. Go onto the sidewalk at the driveway just before the red fire hydrant, then turn right to get onto the path:
Follow the Elm Brook path to where you can't avoid taking a curve to the right. At that point Mongo Brook will be on your left. Keep going till you intersect the RBT, as shown here:
I know you are familiar with the RBT, so I think you will recognize that junction, in case you want to do this in the other direction.
BTW, there is another Elm Brook path, marked by the sign seen at the end of the exit video, that is much rougher. I took it once, it was all rocks and roots - it is the twisty one shown on the Bedford map.