But that only protects you from a motor short or stall, again a pretty rare issue. If you put it between the board and the track on the AC side you do need to determine the total AC load, versus a Motor only DC load.
This may be necessary for the mini cdr, but I have not seen any of Lionels other boards go that bad when a fet/Triac or transistor for motor control goes. Not to say it can't but I have not seen it and I have repaired quite a few 103E units and some of Lionel's new Hi current Reverse boards for conventional engines. LCRUs and DC/AC DRs have been pretty robust. G