Mainline subs on the Shuttle were not infrequent.
At least at the end of the days of the dedicated fleet, back in 2008, the Shuttle used 1-class MD
-88s. I think Delta had 9 ships in this configuration, which was enough to cover the LGA
hourly service (four ships per route each day) with one spare.
If more than one a/c was out of service or on a heavy Mx visit, then a 2-cabin ship was subbed in.