"Do you know what a Jeppesen 10-9 page is? for those who don't, it is a very nice diagram of the airport in question along with (on the back) useable runway lengths including beyond gs.
So that's where I got the info. Anyone who has different info, please post. The runways haven't changed at MDW in a heck of a long time, but as my charts are slightly out of date (this century though!) someone might have newer info."
Thanks. I don't have access to Jepp's for a few more days. This would seem to imply that the Threshhold crossing height of 51 feet for runway 31C is for the displaced threshhold. Possibly misleading to some pilot's using government charts if landing distance beyond the glidepath isn't mentioned somewhere else.