If you look closely at the LV photo, you'll see that the joint was actually installed after the fact. Granted, the concrete was new, but it was sawcut and then attached. The edge of the concrete on top, where it meets the plate, is cleanly cut. It wasn't formed that way.
They just built a maintenance spur, and could test a joint at the end of that beam. Or, at the end of a beam in the garage (I presume...I've never seen them in person) since that concrete is the same age. There's plenty of places to do a little R&D.
The fix then is quite simple. There is no rebar up in the top of the beam where this plate would be inserted. It's only about 3" deep, and is probably just a cap anyway, and isn't a full 3" thick. You score off the concrete, drill in some Hilti expansion anchors and use 2-part epoxy to adhere them. Then you bolt down the plate, and voila, it's literally done. The work could be done and the epoxy cure before they send out trains the next morning, especially if they did it on a night when the parks close early.
As a side note, concrete (if formulated and installed correctly) is remarkably strong and long lasting. Before steel, everything was concrete. There are old buildings standing today that have concrete basement walls, and formed-in-place concrete columns and beams. Having seen WDW's monorail beams, they look pretty darn good - especially after being power washed. The only place they're actually eroding is in the stations where oil drips on them (which is terrible on concrete). There are surface fissures in other places, but those don't cause any structural deficiencies.
