We were just on the Southeast Tours/Yukon Horses tour while on the Summit last week...
Southeast tours works in conjunction with Joanie at Yukon Horses for their tour...They take you by van from Skagway to Emerald Lake--where Joanie supplies the horseback riding, then you return to the ship by van...
White Pass IS the rout between Skagway, Alaska and Carcross, Yukon...you can do it by train or by highway...The train is an old-fashioned RR and that makes it interesting, but you will see a lot of the same scenery by either route...
My daughters love riding horses, which is why we did it...but, for me, and I haven't been on a horse in over 30 years, it was an interesting experience to say the least...my wife had it even worse...she had trouble walking the next two or three days...but it was a unique and special excursion...For older folks, unless they're into horses, I'd say take the train...
BTW, if you do go with horses, definitely do it with these guys and not with the ship excursion...The ship's excursion (at least the one offered by Celebrity) is half the time for the same price...and stays locally in the Skagway area, so you miss the White Pass scenery)
Have fun...