STL is at least mainline (from the last time I checked) so you don't have to deal with the eagles nest bus, but you will likely have to transfer from T4 or T5 to TBIT (or maybe T4 if you're lucky).
I personally wouldn't have agreed to a 51 minute connection anywhere in the first place, and 41 in this situation is borderline impossible, although it shows the minimum connection time as 40 minutes.
it is mainline. I had no choice on the initial 51m connection.