Getting back to the initial inquiry, one thing I don't see mentioned is the wire leads can harder or otherwise deteriorate over time and this could cause them to become less flexible. Additionally, a PO could have found the disintegrated wiring and performed a repair with incorrect wire not designed to stand up to the flexing and/or could have applied solder such that it had flowed further up the wire than expected again reducing flexibility. Ensuring superflex wire (sometimes listed as SW-22 and SW-24) is utilized and properly soldered will likely alleviate all but the more extreme cases.
Good modern locomotives for pulling Postwar operating cars with sliding shoes are not limited to those with speed control like Proto 2 and 3 and Lionel LC+, Legacy, etc.
There are K Line and Williams engines without speed control that are also slow smooth runners, especially pulling a heavy load. I had my K Line GP38 Chessie System diesel pulling 11 Postwar coal dump cars and caboose this morning at a snail's pace without risk of snagging any sliding shoes. Will post a video showing this later. Arnold